Perplexing shower leak via tiled wall

[nadimaj]

Last year this time, a shower leak developed (shower fitted 10 years ago). We have a fairly large shower tray set up sitting on a timber frame. Did the usual re-sealing, fixed the problem for a few months, leak came back. Got in an expert, he said, it could be play in the frame so he tightened it up, re-did the sealant and we were good again for a few months. Fast forward till now, it's been sealed and re-sealed a further 3 times by various people using a variety of brands. Problem disappears for a few weeks at a time but eventually comes back. I then did it with a shower sealing strip. Again, improved the situation for a few weeks, eventually the leak came back. All pipes have been checked and there is absolutely no path for water to leak via the sealant.

One guy suspected the tiles, we re-grouted them but to be honest tiles and grout looked in excellent condition anyway (no missing grout, no play in the tiles).

Last week, I ripped a few bin liners open and attached them to the wall using some sticky pads. It's not 100% water proof obviously, water is still sitting on the seals but guess what? No leaks!!! 20 showers later, not a single puddle. How in the world can you explain this? Perfectly fitted tiles, no visible grouting issues, everything sealed pretty well yet water is getting through? I'm going to fit some cladding to the tiled wall but I just wanted to share the oddest and most bizarre leak issue of my life.

Thanks