Well, I’m not sure what the gain would be.
It probably won’t make any difference to the propriator.
The teen has obviously realized that stealing was a mistake, and that it was wrong. It is thus unlikely to happen again. It seems the teen has taken responsibility for the action on a personal level.
I expect he or she is embarrassed about it, which is natural enough. Does it cross over into being pride and not wanting to admitting wrong-doing? I am not sure.
An important part of restoring the balance of justice is making amends. Making amends to self, to the person wronged, and to God. To God, well, going to confession and completing whatever the penance is is probably the best. To the proprietor - does he require more than the item returned in these circumstances? To self - I suspect that whatever makes amends to the proprietor will also work here.
I think at this point I would discuss it with the teen, making sure I covered all these issues. I might ultimately let him make his own decision - returning the item instead of hiding it shows some spine and maturity, and may deserve giving the decision to the individual after the discussion.
I would emphasize confession and completing any penance given (including apologizing if that was part of it, evenn if the teens own decision was not to.)