We haven't, but there's only 3 of us to keep track of.
DH got lost, though, at
Disneyland when he was a kid. Actually, he didn't get lost, he didn't even move! He was the map-reader that day, and had his nose buried in the map, and his family started moving, and left him behind! He finally noticed, and *stayed put*. He did not move. And I think that's vital. Kids need to stay where they are, not go running around. And parents need to send one person back to the last place they saw the child, so that a child that has stayed put can be easily found!
Hubby was found by, of all people, Captain Hook! CH could see that Robert was missing some family members, and stayed with him until his family finally realized their middle child was gone, and came back.
To this day he will give CH a big hug. He's a villain, but was the good guy that day!
For the big groups, even when it's just the 3 of us we make sure that ONE adult has the main charge. If we need to switch, we say "OK E is with you now, OK?" and wait for a response. When we had our reunion in December with 8 people (including 3 kids), we had to do the same after nearly losing two kids (oh I guess we did lose DS for a minute or two). The 3 year old and my son were standing together in the room when you get off Spaceship Earth, and the 3 year old bolted. My son grabbed her hand and tried to get her to stay there, but she's a strong and determined tiny mite of a thing, and dragged him along. They were found quickly.
And after that we reverted to the verbal "handoff" of kid duties, never allowing a child to just run off to another adult, always requiring them to wait until we could make the hand off obvious.