Where has water come from!

[Caulkhead]

Stripped all the back of the tourer out today, seats trim the lot. Had a puddle under the passenger side rear seat, so decided to investigate.

Did Arctic’s sunroof drain mod years ago, and all is still well there, no damp in spare wheel space or in lining on sides of seats etc. No damp on carpets in passenger space, pleniums all ok. Small rubber blanking plug had come out on wheel arch, it was lying in the seat well, could this be where water had got in when driving in the wet weather? Why is there a hole there anyway? Before I put it all back could there be anything else to check?