Trunk latch failing?
 

All the way from the start when I got this X5, back when it was 51000km young, the trunk lid (the top part) would need to be "shut" for it to latch.

Well, these days I have to shut it harder and harder. Plus sometimes, I fire up the car, start driving, and then the OBC gives me a random "TRUNKLID OPEN" message.

I go back there, and indeed, the two trunk lights are on! This is strange, because I know with certainty that I had shut it previously.

So is this a common issue that the trunklid doesn't latch problem and unlatches itself?

Cheers,
Ian

Yes it is. Look at the thread next to yours: F*ing annoying problem

Maybe you should look into soft-close retrofit mod. Not a problem in post-2003 as they come with soft-close trunklid.