A stray? 'Not likely from any of yours - too soon unless you overlooked a cell when dividing. Perhaps an overlooked cell/hatched virgin from one of your other nucs that got a bit confused/lost?I recently split a colony 3 ways, giving 4 frames each to 2 nucs and 8 frames to the 3rd. After a week, I went into the nuc which housed the original queen. Everything looked good. Found her and plenty of bees and eggs, but under the last 2 frames I noticed a ball of bees in attack mode on SBB. The ball was the size of a golf ball. I removed it and started prying bees away with my hive tool. A queen emerged and flew away. I was kinda surprised to see this queen and don't know what to think of it. I made sure the first queen I found was still present and I closed it up. Any thoughts?
If one shows up "early" in one of the other splits, I'd suspect that's what happened.