I am a pool owner in the UK.  I have an in ground pool.  It is indoors and has a pool thermal cover.  I have done a bucket test and there is almost zero evaporation.  The pool has a liner. The pool size is 26ft x 15ft and when at full level is 5ft deep. The pump, heater and filtration system is all switched off.  We have a leak of about 16 US gallons per day that is proving to be very, very difficult to find.  The water level is now sitting below the skimmers, vacuum point and inlet jets.  I know it is a leak because I have detached the liner from the rail in the four corners and can see water between the concrete wall and the liner - although it is at a lower level than the water in the pool. The only places where the leak could be are the pool light niche, the main drain and the liner.  I have checked the light niche and main drain very carefully using leak detection dye - but no luck.  Can sombody advise me what size a pinhole could be in the liner so I know what I am looking for.  Also, how common is it for the liner seem welds to split.  Any other tips that could be helpful would be greatfully appreciated. Thanks. Gavin.