Have a time machine drive automatically disconnect itself after backup
I suppose you could put together a script which runs a diskutil eject command on the specific volume, then schedule it to run at 4:30 PM each day via cronx or another scheduling app.  When you go to leave at 5:00 PM, the disk has already been ejected.  The drive will always be something like /dev/disk2, so the script only needs to eject the specific volume name.

I would suggest ejecting it at the end of your day versus after a single backup.  This way, you're getting the benefit of multiple backups throughout the day.