I've had this happen twice in the past week with my 2004 GMC Sierra SLE Z71 Ext cab(5.3L). I pull into the parking space, turn off the engine and remove the key. I get out and open the back drivers side door to get my things out. As I'm standing there the horn starts honking and the lights flashing (alarm?). It will not stop until I get back in and put the key into the ignition.
Something else that has happened a few times: Get in, turn the key and start the truck and the radio/clock have reset like the battery has been removed. My presets are not gone, but the clock is back to 12:00.
Has anyone else had this problem, or have a clue as to what's going on? It only has 251 miles on it, but something tells me I may be visiting the dealers shop manager shortly.