Bees living in a barrel

Colonies shouldn't last more than a two or three years without mite treatments. This barrel has been next to my apiary for 3 1/2 years and it's still not dead. I didn't hive them last year because they should have died out. So I'll transfer them to a hive in April. The barrel is falling apart so I was able to lift the lid and look inside. Not too many bees but they're bringing in pollen so there must be brood in there somewhere.

You can see the bung hole from both inside and out.