They're extremely hard to keep... Anything could have happened. They could have been sucked in by a pump intake, a fish could have had them for a meal... If you don't have enough hair algae, they could have died from starvation..
HTH
A couple of years ago... I had gone to paris.. My tank had quite a bit of caulerpa in it. I had bought a rock with it on it, let it take over the right corner of the tank. The person who was watching my tank while I was away, pulled all of the caulerpa out.. I'm not sure what else they did, but...