Same as with assistive equipment and service animals-you shouldn’t even touch somebody else’s instrument without asking, usually. Again, unless some kind of unusual familiarity or intimacy creates an exception
Yes. And also, this is *especially* the case if they are not present. Because some instruments are easy to break accidentally in ways that aren’t obvious. If you touch someone’s instrument when they’re not there, they can’t stop you from doing things that will break it.