To add to the confusion of the very real risk of immoral and unnecessary dental work mentioned elsewhere in this thread - xrays can be legitimately difficult to interpret when looking for small cavities.

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.