That would be the problem! Assuming it was someone that wouldn't mind you sneaking in a stash, they'd be unaware of it and therefore not use it if the need arose. Perhaps a better option would be to prepare a small kit, then have a discussion with the recipient.

I went though this with my brother. He doesn't like clutter and keeps his car immaculate, but that means he has virtually nothing useful if there's a problem. A couple winters ago he came to visit (a very long road trip for him) and arrived in shorts and a T-shirt with no warm clothes or even a blanket in the car! So I gave him a spare Wiggy's 0 degree bag on the condition that he leave it in the trunk, and to his credit he has. So far he hasn't needed it but it could save a life in an emergency.