mounting USB devices

I have an experience about the auto mounting of USB devices which has been going on for
a long time, but continues to puzzle me.

A Debian desktop is running using an /etc/fstab file in which the mount options for
a couple of USB devices are defined. Both devices use the noauto option.

Both devices are already plugged in when the system is rebooted. It has always been
found after a reboot that both USB devices have become mounted, despite the noauto
option.

I understand that during the reboot process th...