Rear 700C tire on hybrid bike flat/deflating seemingly at random
Something subtle is in the tyre puncturing the tubes. It may be rubbing rather than piercing, and it may be imperceptible without pressure, but it's there. To help you locate it, align the logo on the tyre with the valve every time. Then when you find the hole in the tube you know where to look in the tyre (with some margin for error, and you might flip the tube over, so check the other side of the valve too).
As well as glass, wire, screws etc. I've had the threads from a nearly worn out tyre appear on the inside and cause slow punctures, and I've had the inner end of ice spikes wear holes in tubes.
Slow punctures are odd. They can seem to hold for a while then drop suddenly. Whether that's real or just that you notice suddenly, I don't know, but I've had a couple recently do odd things like that