Last night, the alarm went off on my 88 300ce. First time in 5 years. Never heard it before!

My gf and I went out and I think I locked driver door when we got out of the car. When we returned I noticed passenger door was ajar. As soon as I opened it (passenger door), horn started beeping and lights started flashing in intervals that lasted about 30 seconds with breaks of silence spaced in between.

I'm driving and she's reading the manual and finally I pulled over and reset the door lock on driver door with the key and it stopped.

Questions: Would locking the door, with one door remaining ajar (and unlocked) set up this potential alarm triggering condition?
Now that I've reset it with the key, it's in it's "operational mode"?
Once triggered, will the alarm ever go quiet without being reset with a key?

Anyone have knowledge of the workings of the system?