Dome light comes on by itself ?

The dome light will come on automatically if the car detects the FOB beside it. This is for security (you can see in the back of the car before getting in).
I can't see any way it could come on for no reason, other than a faulty auto system or switch (perhaps a switch closes when the car cools off).

Better figure out if it's a fault and get it fixed or you'll have a dead 12V battery soon!

 

I know the interior lights come on when I walk up to the car with the key fob in my pocket - is that what you're talking about? If you hadn't noticed it coming on you would think it had been on all the time.

 

Mine comes on frequently after I pile into the driver seat and then hustle my heavy bookbag into the passenger seat.

I figure that my bookbag maneuver is brushing against the protruding dome light switch, which is 'on' when pushed to the passenger side.

This happens a couple times a month at least.

 

You have a G3.
As the others have pointed out it's probably the SKS (keyfob) activating the lights. It could also be a hatch or door not fully closed or an interior light was pressed on and not pressed back off.
An open door (or hatch) will keep you from being able to lock the car. A lighted dome light might be the culprit. I've jabbed mine on while gathering stuff and things and forgotten to jab it back off before leaving and locking the car.
Since the interior lights come on when you park and shut down....it's easy enough to do.