Did you see it fall off? There is usually a metal clip on the hose that could have rust but nothing that would cause the hose to fall off with no pressure in the system.

Honestly sounds like a way for a shop to earn some easy goodwill with a client who doesn’t know any better. Invent a small problem that can be fixed for little/no cost. To be honest the rusty hammer thing sounds similar. In any case, a hammer left in the engine bay is highly unlikely to do any damage so it’s funny if they found it but not like they saved you from anything.

Your radiator hose was so rusted it fell off?

A foolproof solution is to install battery terminal connections with a quick disconnect. That way you can disconnect the battery when you go out of town and ensure that nothing (dashcam or otherwise) will drain it.

Or just remove the fuse controlling the dashcam.