My dream last night

I had a weird dream last night.
Thing is, I get weird dreams all the time, completely nonsense without any meaning yet they seem so natural when you're in it (I bet that everyone gets those kind of dreams from time to time, if not all the time...).

I was back in Paris and wanted to go to the beach (already a nonsense, Paris has no beach). I wonder why but I had to take my car so I headed for the garage in my swimming trunks and a towel.
Upon arriving at the garage, there were people already gathered chatting. I went in to get my car when I realized I forgot my keys.
Bummer, heading for home to get my keys. That's when one of the people gathered chatting with the others called out for me. He was actually the son of the caretaker (a guy I once "knew" back in elementary school, like...20 years ago, never been friends with him though).
He asked me what happened. I guess I was looking suspicious, coming at the garage in swimming trunks and leaving soon after.
I explained my situation (the beach and the keys) and said that I'd be back soon with my keys to get my car.
I already left when he called out my name again. I came back and I saw him turn around and wave his hand looking far up behind him, in direction of a window in a building. He shouted "WAIT ! DON'T SHOOT ! DON'T SHOOT !" and turned back to me to ask if I could give him a drive when I came back. He had few things to bring to the beach, something about equipments for a concert if I recall correctly.

The dream more or less ends here.
Now if I analyze the few facts present in this dream, some could be explained while others I have no clue about.
I'll start with the ones that could be explained:
- being in Paris: makes sense since I lived there for like 24 years
- going the beach, wearing swimming trunks with a towel over my shoulder: for many years, I've been going to Spain during summer holidays. My parents own a flat near the beach, like 20 meters away (or 65 feet if you prefer)
- my car in a garage: actually, not really my car - my mother is the main driver of that car (the former car of my father) although I can take it whenever I need it. It is indeed parked in that very same garage.
- forgetting my keys: I don't recall ever forgetting my keys though each time, I'm afraid of it so I make double sure I always have them with me.
- the son of the caretaker was indeed a guy I knew back in school and he was indeed the son of the garage caretaker
- something about a concert: a friend blogged few days ago about a concert he attended
- the presence of a sharpshooter in that building: playing too much CoD4 I guess...

Apart from the few specific facts like the keys or the concert, I guess that the background seems rather accurate (because I've already had dreams when the background definitely made no sense at all, with places I've never seen and people I've never met and speaking languages I don't even understand)

Now, for the unexplainable facts:
- why didn't it strike me plain the face that Paris has no beach ?
- why didn't I feel weird about wandering in swimming trunks and a towel in the middle of Paris ?
- why does a guy I've never seen in 20 years just pops into my dream like this ?
- why was there a sharpshooter in that building ? And more importantly, why didn't I say anything about having a guy aiming at me through his sniper rifle ?

Which brings me to the more general question: is there a way to tell I'm in a dream ? If in my dream I could say, "Ok this isn't right, things like that don't usually happen, you're in a dream so wake up !", well I guess I could wake up.