When putting on the winter tyres, I noticed one had gone rather flat over the summer (1 bar, down from 2.5). I pumped it up go 2.5 and let it rest for week in my trunk, it was then down to 1.5 bar. Here's what I tried to find the leak:
pump up to 2.5 bar again, put in bathtub (up to the rim) to look for bubbles, rotate slowly
spray with soap-water mix (as described here), including valve and rim between tyre and wheel
check if valve sits firmly, screwed it in about a quarter turn tighter.
I didn't find the leak. What should I do next to fix the tyre, or do again but differently?