You should delete all your personal data and then leave a mobile in the support store, I won't leave my phone unblocked to anyone.
Depending on your level of carefulness… I would consider your phone not secure if it’s confiscated and you’re not in the same room as it.
I think running a system restore should make the phone reasonably safe?
(btw, this is partially the logic of why Apple wants to forbid uncertified third party repairs)