There is a serious bug in Basecamp, where sometimes newly imported caches are in a weird intermediate state between "Found" and "Not found".
They will have the "not found" icon, but when you open them you will see a text that says "Found at ..." with some apparently random date and time. It will also not be possible to trigger the "found" status of such caches, as the button is missing. When I move such a cache to my Oregon 300, on the device the cache will appear as "found".
This stumped me for a long time, but now I know where the apparently random date/time comes from: Those are found dates of other users (often from one my "friends" on the GC.com platform). I investigated the GPX file used to import the caches in question, and the GPX does NOT contain the (sometimes very old) logs where these dates/times come from.
So my strong suspicion: I had probably imported these caches before and deleted them again, but somewhere in Basecamp's database, the old logs or old found date/time still lingers. Why it uses the date/time from other people, no idea.
I tried deleting the "GeocacheLogs" folder (after backing up the entire Basecamp database of course), but while it made all logs on all caches disappear, it did not fix this particular issue. So the "lingering" must happen elsewhere.