Mysterious transmission leak — happens only sometimes

This has been stumping me for a while and I can't figure out why it's happening. I had the transmission rebuilt a few years ago and it's seemed to drive fine since. But on a few occasions, it just dumps all its fluid out. Last month I drove it on a long cruise and didn't spill a drop. Yesterday I moved it out of the garage and back in, and this morning there was a puddle of fluid underneath it. This has happened several times and I can't find a pattern. Anyone have thoughts?