THe smoke pellet trick might work, and it's certainly worth a try, but my gut feeling is that the smoke will be poiuring out of the door and window seals long before it finds it's way out of the roof. But certainly try it first.
If it does not work, you can try flourescent spray crack detector, like the stuff used on welding joins and after grinding things like crankshafts. You spray it on, wipe or rinse it off then shine a UV light on it. Any cracks will be immediately obvious.
I've seen this used on fibreglass dingies - hit a rock and the hull starts to leak water. You patch the damage but the leak continues. The problem is that an impact can easily cause long hairline cracks that are all but invisible. Water gets in through the crack and works its way by capilliary action through the fibre matting until it finds a crack or scuff on the inside it get out through.
This may be what has happened to your roof - something has hit the roof, cracked it and the water is getting in, then finding a weak spot to get out. Don't be suprised if the ingress point is a few feet away from where it is coming out.
If you do find a crack, try this stuff to seal it:
https://www.amazon.co.uk/Captain-Tolley ... B003T6EJ9AIt is made for marine applications so is perfectly suited to fibreglass and works wonders.
2011 SWB medium roof 2.2l Mk7 115 T300 in Frozen White. Stealth Camper converted.
"It's better to burn out, 'cause rust never sleeps..."