I had a ford contour that I got cheap to flip. Was walking by with it of course shut off and the thing was buzzing, an electrical sort of buzz. Traced it to the ignition module. Back traced it to a relay that powers more stuff when the key is on. Switched it with the horn relay and had no more problems. (figured if I did, I'd hear about it
Also check your glovebox light, and anything in the vicinity of the sliding door(s), or any other fufu stuff like remote control door openers etc. I'm amused/chagrined at the variety of stuff people are chiming in with that could be the cause.
On GMs you can cheat the passkey antitheft by IIRC leaving the ignition on for an hour or doing some other lengthy dealie. Might check around online to see if there's a redneck way to get the key relearned.