Rear 700C tire on hybrid bike flat/deflating seemingly at random

I have a hybrid bike and I am having a very strange problem with deflating tires.
My bike has 700C 35/43 tubes

I have changed 3 tubes in the last 2 weeks because something very strange is happening.

Tube 0

• That I had for a long time punctured
• I found the small hole

Tube 1

• I change the tube and pump it to the suggested pressure of 80-85psi.
• I ride it for 1 day, check the pressure, it stays at 80-85
• I leave it overnight, check the pressure, ride it for 1 more day, check the pressure, everything is fine
• I leave it overnight, the tire is completely flat. I

Tube 2

• I change the tube and pump it to the suggested pressure of 80-85psi.
• I ride it for 1 day, check the pressure, it stays at 80-85
• I leave it overnight, check the pressure all is fine, ride it to the supermarket, pressure is fine. Go in for 1-2 hours, come out, tire is flat.
• I pump up the tire, ride home, check the pressure, everything is fine.
• I leave it overnight, check the pressure in the morning, everything is fine.
• ride to the office, check the pressure, everything is fine.
• come out for lunch 3 hours later: tire is flat.

Repeat with one more change of tube. Same result. Completely random deflation. I visually and tactilely inspected the rim, the tire, the tape and cannot find any problem. The tire is not new, sure, but it is not bald either. Should I replace the tire? The whole rim/wheel (more expensive, also, the cassette is difficult to remove and I have never done it)? The whole wheel with cassette? The whole bike?

I am somewhat technically minded so I tried to understand but my observations essentially are useless as I can find no logical pattern. Has anyone had any similar experience?

Thanks!