Now you have found it you have the choice of removing it, remember if you reconnect it your battery you will have a constant drain,If you re-register it with the organisation that it is assigned to the device you will have to pay a annual subscription and prove you own the car .
I have never found mine I suspect it is somewhere in the floor space but let sleeping dogs lye is my motto, in your case searching for it as been a good outcome in finding the possible rust scenario.
Get that cleaned up and sprayed with wax as soon as .