A few weeks ago I noticed that my bike tube slowly leaked out air over the course of a day or 2. I bought a new tube and it does the same thing. I checked in water and saw no leaks.
9/10 it’s a tiny sharp object like a sliver of wire stuck in the tire. Take the tube out of the tire, pump it up really big, run the tube through your hand until you feel where the air is coming out. Then look at the tire in that spot and find the thing that poked a hole in the tube.
Alternatively, switch to tubeless and don’t worry about small punctures anymore.