Ideally you do want to get even small ones filled as a preventative measure. However IME trustworthy dentists will acknowledge the potential for false positives on less obvious xrays upfront and offer a choice of either probing the area with a very fine drill first or simply defer work altogether and monitor it on each check-up.
Point is, in your case it could be an untrustworthy dental practice (suggesting unnecessary procedures), or the original xray was ambiguous and there was never anything there, or the dentist was poor at interpreting it. Either way if it was that minor they should have made it clear - i'd recommend scepticism next time, feel free to interrogate them, they are doctors and usually welcome it... if they are unconvincing and defensive, go elsewhere.
To add even more confusion, this is mostly untrue. Fillings don't last forever, so prematurely filling "starts the clock" because the filling will eventually fail and every time a filling is made, more healthy tooth must be removed. (I say mostly untrue because mercury amalgam fillings really can last multiple decades, but these have fallen out of favor).
I went back two years later for another checkup/cleaning. They didn't find any cavities.
Maybe it healed itself? Apparently that's possible via re-mineralization. Either way, if that was possible, then perhaps they shouldn't have been so eager to drill and fill?