Couldn't someone:
a) physically remove your hard drive
b) mount it in a different machine
c) find your SD checking script
d) figure out what key file you're checking for
e) write that key file to a new SD card (or otherwise circumvent the check)
This assumes that your HD isn't fully encrypted, in which case you'd be pretty well protected. It does sound like a nifty way to auto-logout quickly though...