I assume by "GC" you mean "Geocoin", not "geocache", 'cause that wouldn't make ANY sense.
If you're going to remove the item from the cache, simply file a 'retrieve' log as you would if it were in someone else's cache. If you're going to leave it in there, file a 'discovered' log.
You would do these from the item's webpage as opposed to doing it on the cache's webpage because you're not interacting with the cache at all, log-wise.
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If, however, you're talking about logging a TB INTO your cache that you found IN there but not registered as BEING in there, then two things:
1. Give it a little time for the person who dropped it to log it. Couple of days, at least. Not everybody logs 'live'.
B. After a while, contact the person in who's possession it's listed. Contact them and gently remind them to log it in. You CAN offer to do it yourself with their permission - file a "GRAB" log, then a "DROPPED OFF" log. I don't think I'd do that, tho.
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Or, are you talking about something else?
Perfect, thank you!