I am currently having problems with the stock alarm on my Canadian 2006 Mazda3 GT going off at random times. Because it specifically does it often when I unlock/lock the car with the remote and then open the door, I suspect it is the door sensor.

I don't however know where/what the mechanism is that determines if the door is open or not. Can anyone enlighten me so I can do more personal testing to confirm my diagnosis?

 

I think the switch is on the actual door latch mechanism, to get to it the door panel comes off, and you may have to pull the inner door panel off as well (PITA to do). There may be an access hole in that inner panel, but I don't recall.
You could have some broken wires in the door harness as well, you'll see that when you take the panel off.