We have had a similar problem with our Lay Z Spa St Moritz after refilling the tub after Winter break. The cold water filled to the required level and stayed at that level for 36 hours whilst we put in the chemicals and filtered. When we switched on the heat everything was fine until we opened the lid next morning and half of the water had gone. There was a lot of condensation around but after draining and checking every inch of the tub (inside and out) we couldnt see any puncture. It was suggested to us that the drain plug could be the issue so we cut down a polythene freezer bag and dropped this into the hole and pushed in the plug to seal. A week later and there has been no further leakage. As for permanently sealing that may be a good idea and is something we will also look at. We are just pleased to use the tub again. Let us know how you go on if you do go ahead with sealing the plug hole. BTW the plug unit is very flimsy and considering the amount of water and pressure you would think it would click firmly shut or twist and turn with a decisive click to lock. It just limply goes into place with very little conviction if thats the way to describe it. Regards David.