I don't really know how long it's been going on. About a week or so ago the error message finally piqued by curiosity with, "I wonder what that means?" Before that, it didn't register, so I don't know when it first appeared.
After I finally noticed it, that's when I started trying to figure out what it meant. Here the output of sudo dmesg | grep -i usb:
jim@jim-pc:~$ sudo dmesg | grep -i usb
[sudo] password for jim:
[ 0.341245] ACPI: bus type USB registered
[ 0.341259] usbcore: registered new interface driver usbfs
[ 0.341266] usbcore: registered new interface driver hub
[ 0.341289] usbcore: registered new device driver usb
[ 1.327080] ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
[ 1.327164] ehci-pci 0000:00:12.2: new USB bus registered, assigned bus number 1
[ 1.341179] ehci-pci 0000:00:12.2: USB 2.0 started, EHCI 1.00
[ 1.341224] usb usb1: New USB device found, idVendor=1d6b, idProduct=0002
[ 1.341225] usb usb1: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 1.341226] usb usb1: Product: EHCI Host Controller
[ 1.341227] usb usb1: Manufacturer: Linux 4.10.0-21-generic ehci_hcd
[ 1.341228] usb usb1: SerialNumber: 0000:00:12.2
[ 1.341322] hub 1-0:1.0: USB hub found
[ 1.341499] ehci-pci 0000:00:13.2: new USB bus registered, assigned bus number 2
[ 1.357176] ehci-pci 0000:00:13.2: USB 2.0 started, EHCI 1.00
[ 1.357230] usb usb2: New USB device found, idVendor=1d6b, idProduct=0002
[ 1.357231] usb usb2: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 1.357232] usb usb2: Product: EHCI Host Controller
[ 1.357233] usb usb2: Manufacturer: Linux 4.10.0-21-generic ehci_hcd
[ 1.357233] usb usb2: SerialNumber: 0000:00:13.2
[ 1.357317] hub 2-0:1.0: USB hub found
[ 1.357480] ehci-pci 0000:00:16.2: new USB bus registered, assigned bus number 3
[ 1.373174] ehci-pci 0000:00:16.2: USB 2.0 started, EHCI 1.00
[ 1.373214] usb usb3: New USB device found, idVendor=1d6b, idProduct=0002
[ 1.373215] usb usb3: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 1.373216] usb usb3: Product: EHCI Host Controller
[ 1.373217] usb usb3: Manufacturer: Linux 4.10.0-21-generic ehci_hcd
[ 1.373217] usb usb3: SerialNumber: 0000:00:16.2
[ 1.373303] hub 3-0:1.0: USB hub found
[ 1.373407] ohci_hcd: USB 1.1 'Open' Host Controller (OHCI) Driver
[ 1.373478] ohci-pci 0000:00:12.0: new USB bus registered, assigned bus number 4
[ 1.437233] usb usb4: New USB device found, idVendor=1d6b, idProduct=0001
[ 1.437235] usb usb4: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 1.437235] usb usb4: Product: OHCI PCI host controller
[ 1.437236] usb usb4: Manufacturer: Linux 4.10.0-21-generic ohci_hcd
[ 1.437237] usb usb4: SerialNumber: 0000:00:12.0
[ 1.437369] hub 4-0:1.0: USB hub found
[ 1.437524] ohci-pci 0000:00:13.0: new USB bus registered, assigned bus number 5
[ 1.501243] usb usb5: New USB device found, idVendor=1d6b, idProduct=0001
[ 1.501244] usb usb5: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 1.501245] usb usb5: Product: OHCI PCI host controller
[ 1.501245] usb usb5: Manufacturer: Linux 4.10.0-21-generic ohci_hcd
[ 1.501246] usb usb5: SerialNumber: 0000:00:13.0
[ 1.501380] hub 5-0:1.0: USB hub found
[ 1.501535] ohci-pci 0000:00:14.5: new USB bus registered, assigned bus number 6
[ 1.565223] usb usb6: New USB device found, idVendor=1d6b, idProduct=0001
[ 1.565224] usb usb6: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 1.565225] usb usb6: Product: OHCI PCI host controller
[ 1.565226] usb usb6: Manufacturer: Linux 4.10.0-21-generic ohci_hcd
[ 1.565226] usb usb6: SerialNumber: 0000:00:14.5
[ 1.565391] hub 6-0:1.0: USB hub found
[ 1.565524] ohci-pci 0000:00:16.0: new USB bus registered, assigned bus number 7
[ 1.629221] usb usb7: New USB device found, idVendor=1d6b, idProduct=0001
[ 1.629222] usb usb7: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 1.629223] usb usb7: Product: OHCI PCI host controller
[ 1.629224] usb usb7: Manufacturer: Linux 4.10.0-21-generic ohci_hcd
[ 1.629224] usb usb7: SerialNumber: 0000:00:16.0
[ 1.629356] hub 7-0:1.0: USB hub found
[ 1.629458] uhci_hcd: USB Universal Host Controller Interface driver
[ 1.629521] xhci_hcd 0000:01:00.0: new USB bus registered, assigned bus number 8
[ 1.685200] usb 2-1: new high-speed USB device number 2 using ehci-pci
[ 1.741195] usb 3-2: new high-speed USB device number 2 using ehci-pci
[ 1.835802] usb 2-1: New USB device found, idVendor=0bb4, idProduct=040b
[ 1.835804] usb 2-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 1.835805] usb 2-1: Product: Android Phone
[ 1.835805] usb 2-1: Manufacturer: HTC
[ 1.835806] usb 2-1: SerialNumber: FA5CFSX06351
[ 1.891555] usb 3-2: New USB device found, idVendor=050d, idProduct=845a
[ 1.891557] usb 3-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 1.891558] usb 3-2: Product: Belkin USB Wireless Adaptor
[ 1.891558] usb 3-2: Manufacturer: Manufacturer Realtek
[ 1.891559] usb 3-2: SerialNumber: 00e04c000001
[ 17.588098] xhci_hcd 0000:01:00.0: USB bus 8 deregistered
[ 17.653212] usb-storage 2-1:1.1: USB Mass Storage device detected
[ 17.653300] scsi host5: usb-storage 2-1:1.1
[ 17.653368] usbcore: registered new interface driver usb-storage
[ 17.654362] usbcore: registered new interface driver uas
[ 17.936178] usb 7-3: new low-speed USB device number 2 using ohci-pci
[ 18.147722] usb 7-3: New USB device found, idVendor=413c, idProduct=2003
[ 18.147725] usb 7-3: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 18.147728] usb 7-3: Product: Dell USB Keyboard
[ 18.147729] usb 7-3: Manufacturer: Dell
[ 18.320157] usb 7-4: new low-speed USB device number 3 using ohci-pci
[ 18.526583] usb 7-4: New USB device found, idVendor=1241, idProduct=1177
[ 18.526584] usb 7-4: New USB device strings: Mfr=0, Product=0, SerialNumber=0
[ 18.546779] usbcore: registered new interface driver usbhid
[ 18.546780] usbhid: USB HID core driver
[ 18.548038] input: Dell Dell USB Keyboard as /devices/pci0000:00/0000:00:16.0/usb7/7-3/7-3:1.0/0003:413C:2003.0001/input/input2
[ 18.604282] hid-generic 0003:413C:2003.0001: input,hidraw0: USB HID v1.10 Keyboard [Dell Dell USB Keyboard] on usb-0000:00:16.0-3/input0
[ 18.604394] input: HID 1241:1177 as /devices/pci0000:00/0000:00:16.0/usb7/7-4/7-4:1.0/0003:1241:1177.0002/input/input3
[ 18.604476] hid-generic 0003:1241:1177.0002: input,hidraw1: USB HID v1.10 Mouse [HID 1241:1177] on usb-0000:00:16.0-4/input0
[ 23.269095] usb 3-2: r8712u: USB_SPEED_HIGH with 4 endpoints
[ 23.269514] usb 3-2: r8712u: Boot from EFUSE: Autoload OK
[ 23.700157] usb 3-2: r8712u: CustomerID = 0x0000
[ 23.700159] usb 3-2: r8712u: MAC Address from efuse = 08:86:3b:92:76:e2
[ 23.700160] usb 3-2: r8712u: Loading firmware from "rtlwifi/rtl8712u.bin"
[ 23.700208] usbcore: registered new interface driver r8712u
[ 25.318721] audit: type=1400 audit(1495017618.212:2): apparmor="STATUS" operation="profile_load" profile="unconfined" name="/usr/sbin/ippusbxd" pid=976 comm="apparmor_parser"
[ 3001.870462] usb 2-1: USB disconnect, device number 2
[ 4728.523819] usb 2-1: new high-speed USB device number 3 using ehci-pci
[ 4728.672686] usb 2-1: New USB device found, idVendor=0bb4, idProduct=040b
[ 4728.672690] usb 2-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 4728.672692] usb 2-1: Product: Android Phone
[ 4728.672694] usb 2-1: Manufacturer: HTC
[ 4728.672696] usb 2-1: SerialNumber: FA5CFSX06351
[ 4728.673817] usb-storage 2-1:1.1: USB Mass Storage device detected
[ 4728.674080] scsi host5: usb-storage 2-1:1.1
[ 4729.323120] usb 2-1: USB disconnect, device number 3
[ 4729.591742] usb 2-1: new high-speed USB device number 4 using ehci-pci
[ 4729.740622] usb 2-1: New USB device found, idVendor=0bb4, idProduct=040b
[ 4729.740625] usb 2-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 4729.740628] usb 2-1: Product: Android Phone
[ 4729.740630] usb 2-1: Manufacturer: HTC
[ 4729.740632] usb 2-1: SerialNumber: FA5CFSX06351
[ 4729.741985] usb-storage 2-1:1.1: USB Mass Storage device detected
[ 4729.742262] scsi host5: usb-storage 2-1:1.1
[ 4731.545627] usb 2-1: USB disconnect, device number 4
[ 4731.871556] usb 2-1: new high-speed USB device number 5 using ehci-pci
[ 4732.021059] usb 2-1: New USB device found, idVendor=0bb4, idProduct=040b
[ 4732.021062] usb 2-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 4732.021065] usb 2-1: Product: Android Phone
[ 4732.021067] usb 2-1: Manufacturer: HTC
[ 4732.021069] usb 2-1: SerialNumber: FA5CFSX06351
[ 4732.022415] usb-storage 2-1:1.1: USB Mass Storage device detected
[ 4732.022631] scsi host5: usb-storage 2-1:1.1
jim@jim-pc:~$