I found money in the pocket of a pair of pants I was trying on in the dressing room of a clothing store. It was less than $100. I didn’t know what to do with it because I suspected that the sales clerks were stealing and concealing it in the pocket of a pair of trousers on the rack (the one I tried on). I didn’t think to ask to see the manager. Did a previous customer put it there when trying on the same pair of trousers? I didn’t know. It might have happened that way. I exited the store with the money. Was this a sin?
Not only did you not know the origin of the money you found, but you suspected that it was stolen money. (And, no, it is not a reasonable supposition that a customer would put his own money in a pair of trousers he was trying on to decide whether or not to buy the trousers. Who puts his own money in clothes he has not yet bought?)
So, in this case, what you did was to keep money you had reason to believe was stolen, not “lost” (which makes the matter more serious). I don’t know why you wouldn’t have thought to ask for a manager when you found the money, but that is what you should have done and then turned the money over to that person. As for what you should do now, I can only recommend that you mention this the next time you go to confession and ask the confessor what kind of restitution would be appropriate.