This was downright strange with my Subaru

Could be related to your security system, or a bad relay/wiring connection somewhere.

On my wife's 03 Outback, the security system can be activated by hiting the door lock button when the door is open. If we are both exiting the vehicle and she is being particularly slow, I HAVE hit the button, closed my door, then, when she closes hers, the alarm goes off.

Dunno why you wouldn't here your horn honking though.

If hitting the emerg. flasher button turned them off, then I doubt it was the security system and rather some fluke with the button itself. Had the car been detailed or cleaned recently? I COULD see a little used button like that one being depressed, but not haveing clean enough contacts to work until some vibration causes it to make good contact. It may have been depressed for days - just waiting for enough shock to complete the circuit. I'd be tempted to just exercise it on/ off a few dozen times, then just monitor for future problems.