Thats why you always have an empty bottle close by for just such an occurence. If some idiot later picks it up and drinks it, thats just a hilarious (for you) bonus.
I found a nice usb cable and a mini flashlight under a booth once. Asked the other guys playing and the club owner about it. Lost & found, turns out, so fair game.
I played a gig in Florida once and realized I’d lost the new usb cable for my audio interface and the mini flashlight while setting up.
I ended up plugging into the back of the mixer lit by the weak light in my wristwatch and used a coat hanger to connect to my audio8.