I think after a few months, the best theory is still Anthony.
I think it would have been too suspicous for Fredo to be out of the house. Couldn't have been him.
More than likely Rocco - after Michael sends him out. Does Rocco re-appear in the movie? I'm trying to remember...
OR - it could have been Johnny Fontaine, with greasy hair and olive oil charm, stumbling around the woods drunk, about to sexually attack the assasins - realizes his mistake (or not) and kills them out of embarassment!