I learned in taking my 609 certification test that topping off a known leaking R12 system is NOT against the law. Many shops tell people this to get them to do more major work or converting. THEY are violating the law in telling you this. The Better Business Bureau does not like them telling you this.
It is possible that this was just the normal leaking of refrigerant that happens over time. A minute amount of refrigerant leaks from all systems.
It is also possible that it is a more major leak. If one can topped it off and it is okay, you need to just wait and see. If you indeed decide to look for and repair the leak. Put in some UV dye, wait a week or two and use a black light and yellow glasses to look for a leak. If you find one, take the car to an a/c shop and have them recover the R12 before repairing the leak. They will even pay you for it if it is not contaminated.
You are DEFINITELY on the right track.