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

I understand that during the reboot process the system issues the ‘mount. -a’ command,
which mounts filesystems defined in /etc/fstab that are not already mounted, except for
those filesystems that use the noauto option.

So what causes the two USB filesystems to be mounted ?

I know that the system contains the usbmount facility, but I understand that usbmount
automatically mounts filesystems when they are plugged in. Usbmount would not automatically
mount filesystems which are already plugged in.

Can someone please help to solve this problem. Thanks.