My car alarm has gone off three times by itself. I started it up after the first time, checked all doors and windows, switched off the engine and then it did it again. After the third time I left it unlocked, alarm lights were flashing after a minute but no noise. Then offside indicator on door was flashing, everything has stopped now but the interior light is on. What’s possibly causing this? 
PS, the interior light has just gone off, then on again. Weird

Sometimes the alarm goes off when the car battery is getting a bit flat.

But if the alarm still went off when it was unlocked then I think there's a bigger fault in this case.  I would be looking for water ingress and rodent damage around the wiring loom, BCM and alarm sounder.

I had this once recently, I eventually found the boot hadn't closed fully even though it looked fully closed. It set off a couple of times in the early hours. I wasn't popular.

This happened to me,

 

Turned out it was the bonnet release switch inside the bonnet latch

I only realised what was causing it when the bonnet open warning showed while driving. A new mechanism from for sorted it

 

I think unplugging it have also worked

1 hour ago, Fkuk said:

This happened to me,

Oooo, did you have to see a doctor ? 🤣