What happened is your bees absconded. New packages do that actually. Why? it can be a # of reasons from being disturbed to much, some nasty scent to them, or a few different reasons.
Those other bees 'could' be your bees..... they are some bees.... so why not? In any case, if you can capture the swarm, do it. Is this a 'new' hive box/frames/etc? no guarantee they won't leave again, but no promise they will either.
you can put in some frames that are drawn out, or rub leaves/etc. inside the frame or rub lemon grass in it, and they tend to like it better. still no guarantee, but it helps I think. when I build new hives for myself, I always fill them with leaves and let them sit outside for at least a few days personally. Also if the hive is in a hot spot they might not like it to much/etc. entrance is good to have morning early sun, no fresh stinky new paint on the outside/etc.