My theory is pretty weak.

The fob is an RFID system similar to that used in material management. Does that store use RFID tags for inventory and theft prevention? If so, maybe they have a transmitter on an adjacent signal and it's interfering with your fob.

You might try for the next week going everywhere but that store and if it doesn't happen again, go back and see if it repeats. Either way, let us know what happens!

Just my $0.02

Larry