Tune2fs "mountcount" and USB drive power-up filesystem checks

Looking to confirm true nature of behaviour.

I use an External USB drive for backups.

Using tune2fs, I have specified a mountcount value of 20 for each of the partitions on that drive. I can't remember how, but I did specify the individual count on each partition so that the fsck would be staggered (not all at the same boot) if I always mount all those USB partitions every time.

QUESTION:
If the partition's "mountcount" dictates that this power-up event would reach the count at which an fsck should be performed,

  • with no auto-mount directive in place, does the simple act of plugging in the USB drive trigger the fsck action ... before any mount directive has been given?           or

  • does the OS perform the fsck only as a pre-requisite first step during the performance of the mount command?