I would say it really depends on the firearm as well. If I were looking at a WWII vintage 1911, an old Colt SAA, or an early Winchester lever action, I would definitely be sure to ask first before each move. "May I take a look at this?" "Is it OK for me to open the action?" Etc.
Now, if it was just a Glock or a police trade in revolver of some sort I would first ask for permission to look at it but after that I probably wouldn't think to ask about opening the slide or swinging the cylinder open for a look around.
Not to point things back at you (since I wasn't there and I really don't know you) but perhaps you were handling something you shouldn't have been handling or handling it in an inappropriate way.

