About 4 times in the last month, I would go down to my car in the parking lot at work and unlock the front drivers side door to get something out of the back seat. I would then close the drivers door but leave the back door open while I'm getting whatever I was there for. It so happened that I would end up talking to somebody and after a few minutes I would hear a "clunk" and the doors would lock on their own. My key fob hasn't worked for the last three years and when this happened this morning I didn't even have my fob with me.
Has anyone else ever experienced this?
When I park at home and leave the car unlocked it remains unlocked. It seems to only happen when the drivers door is closed and the rear door remains open for 5 minutes or so.

Thanks
Neil