This can correctly happen for a few reasons:
- The object isn't a "hidden object"; it's an object that another bear will give you.
- The hidden object is no longer on the island.
- Perhaps it got moved when the island regenerated itself, in which case you may get it back in the mail the following day.
- You picked it up and it's now in your backpack or Storage.
- A bug deleted the object. (The bug is obviously not correct behavior, but the fact that Ranger Bear won't give you a hint is correct.)
- The hidden object was ever picked up, even if you put the object back exactly where it was originally or the game automatically dropped it because your backpack was full.