So, now, after reading this thread, I have been more observant.
Today, I got back in my Denali after a car wash...and got 3 beeps before I started it.
Here was the sequence:
1) Enter vehicle at home, after it sat all night in the garage -- normal, the vehicle did its normal "wake up" when I got in the garage (interior lights on, GMC logo on the ground at the liftgate, and heard the fuel pump prime), no chirps
2) Start vehicle in garage and drive to car wash.
3) Put vehicle in neutral at car wash, turn off engine (with me remaining IN the vehicle)
4) At end of was tunnel, start engine, put in drive, drive out of tunnel to the interior, wipe down, vacuum bays
5) Vehicle in Park, shut off engine, open driver door, exit vehicle w/ fob in my pocket.
6) Car wash people vacuum, wash windows, etc, close two rear doors, rear liftgate, and passenger door, never closing driver door, finally signal me when done
7) I enter vehicle with fob in my pocket, close driver door...3 chirps (likely signifying that the fob was left in the vehicle).
8) Start Vehicle, put in Drive, drive to work.
I think this makes sense, since to the vehicle, the actions in #5, 6, 7 would look to the vehicle like I left the FOB in the car, and closed the door with the engine off.