The battery will drain after 2 or 3 days if the car is not driven. Battery is fine; it has been checked by two different establishments. Nothing is left on (vanity mirror lights, trunk lights, etc).

Try looking for a parasitic drain.

I had this same exact problem. Bring it to the dealer if you're still having the problem. My C70 ended up needing a software upgrade. Something about the satellite radio module trying to find a signal. I don't even have the subscription active but the car probably uses the same system for the navigation. In any case, I just left the car for a whole week and it started up no problem when I got back.

Sun visors!!!! They break internally and drain the battery. No one bothers to check them

I know you posted a while back, but the drivers side sunvisor lights are infamous for staying on. Even when closed, you just can't see the light on. This will drain your battery. It's suggested to disconnect the light.