DukeAMO
Contributor
I've used a bolt snap a number of times to tap the tank when that was what was handy! (I've seen people carry a little spoon in their pocket, too. The point is merely to reach around and tap the tank with something to make noise. There are those elastic tank bangers that you strap onto the tank, too, but people forget to get them off the tank and end up not having them for the subsequent dive....
I agree. I've taken to clipping an extra bolt snap to one of the small front D-rings on my BCD, and just leaving it on there. Once every few dives, I've used it to get someone's attention. It only costs a couple of dollars, and it's super easy to find by touch, unclip, use, and reclip.
And yes, the round tank bangers on a rubber bungee are good at getting left behind on tanks and nothing else. It's too hard to find/reach the darned things.
I love how alert and responsive your buddies were!