a swarm moves into our old postbox every year and because bees are threatened we keep away from that part of the garden until they move out once they did become a problem and we got a beekeeper to remove them.
Nothing to worry about. If you do spot a hive, don't burn it down or anything, just get a beekeeper to remove it as maumau suggested. There's probably a tree in flower nearby or a swarm just moving over the area (they tend to rest for a while in the process, sometimes a few days).