Inexplicable tyre deflation - no puncture, valve OK

Backing out of the drive yesterday I realised I had what I thought was a puncture. Fortunately still being at home it was a quick job to remove the affected wheel and replace it with the spare tyre. (Sometimes you can be grateful for being old school). When I got home I carefully looked round the tread and both sidewalls but other than the odd bit of grit there was nothing. The pressure in the tyre was zero, so absolutely flat. Being tight with my money I avoided rushing straight off to the local tyre dealer for a pair of new tyres - the tread is down to about 3mm on both sides so a puncture repair would been better spent on a new tyre and as I like to keep the tyres on the same axle as close to identical as possible it would have been a case of replacing both tyres.

I have a large capacity plastic trug, I half filled that with water and, after partially inflating the tyre (about 25 psi), I was able to rotate the wheel/tyre through the water a section at a time. After three rotations there was still no sign of a stream of bubbles, not even from the valve, which was my second suspect after a puncture. I inflated the tyre to 40psi (I usually run it at 33psi) and after 24 hours this afternoon it was still holding at 40psi.

I've now refitted it back on the car but I'm bemused as to why it should fully deflate - it was sat under the car port after I used it the previous day. I even wondered about someone letting it down as a prank but firstly "Why?" and if you did that would you bother to replace the dust cap? And nothing was picked up on the security camera that covers the drive and car port, although that's not a guarantee as despite being a Tapo model I've found it not to be 100% reliable, just better than nothing.

As far as I can remember I've not had a puncture repair in this set of tyres and if a repair had failed I can't believe it would self repair. There's no kerbing (curbing) on the wheel that might suggest a failure of the seal between the wheel and the tyre. I'm totally bemused.

Has this ever happened to anyone else or has anyone any thoughts on what might have happened?