Well, have you looked at them to see what is going on? There are such things as false swarms, but they usually precede the real thing by only a few days. Do you mean that thousands of bees pour out of the hive at once, then their excitement subsides and they are returned to the hive a couple of hours later? We always called them false swarms and it usually meant that the queen wasn't quite ready to fly. On a few occasions they never did swarm, but most of the time a swarm would erupt 3-5 days later.

Probably it is not a swarm, because swarms come back at the hive when the queen can´t fly (if you have cut a wing).
If your bees are flying in the evening around the hive perhaps are cleaning the intestines.