When I was young and stupid, I was used to playing in sessions where it was quite acceptable to pick up and play an unattended instrument. However we all knew each other, and none of us had particularly valuable or special instruments. Then I visited another area and got into a session, and picked up a spare mandolin. The owner was surprisingly reasonable about it, but did request that before I borrowed his antique Gibson it would be nice to ask.

I was suitably chastened. However I hope I treated it with respect.

Now I'm older (although probably still stupid) I would be more careful. If for some reason I've turned up without an instrument, I would ask before borrowing, try to demonstrate that I know what I'm doing, and would never re-tune without permission.

But I wouldn't let anyone borrow my Guild guitar, although it helps that I use a tuning even more obscure than DADGAD. And don't even ask about my concertinas...