If you've exhausted all water ingress sites on your side then surely it must be your neighbours side. Evidence supports it (dry your side, damp thier side, you've checked your side etc). Again, don't underestimate how far water can capillary along brick work, pool, then travel again.
Is it your neighbours drain? Get up and offer to do some work for them, just enough to find the source.
Is it your neighbours drain? Get up and offer to do some work for them, just enough to find the source.