The Key Fob is a strap with a ring that can be found on right side of the Belt hanging. While not used much, it was used in Knock, Knock to hold the P.K.E. Meter for Egon.
The key fob didn't appear in Extreme Ghostbusters, even in the two part Back in the Saddle, Part 1 or Back in the Saddle, Part 2.
A Key Fob in use holding a P.K.E. Meter
in place while Egon walks