What Could Be Causing This

I agree, you need to get a professional in there to get rid of them and find the opening they're getting into. It needs to be sealed with expansion foam or something so they don't keep getting in.
Good luck :)

I would call someone who knows what they are doing. I lived in an apartment complex for a while and in the breezeway some bees made their hive, they had an exterminator, construction worker and a bee keeper come together to solve the problem. They too had to get to them from with in the building's walls, turns out they had a pretty HUGE nest, nearly 6'x6' in the wall the bee keeper was happy to help because he got the queen and 90% of the hive or something like that to take back to his farm. This may be what you guys have to do. The construction worker used some sealant to prevent this in the future. This is normal for Texas, my apartment was in Tx when this happened and it was not the first building it happened to in the complex.

Are you sure they are honey bees? There are a variety of wasps and hornets that dig in behind siding and make their nests. An inspector should pick this up on the home inspection, or the homeowner needs to provide proof that a licensed exterminator took care of the problem. Just shooting some spray up in there isn't going to take care of it.

We have a large, wooden play structure in our backyard that attracts carpenter bees each summer. They look like honeybees to me, but we did talk to a professional about it. So easy to treat! We buy an insect spray (not sure what it is, because my husband purchase it) and spray all over the play structure. The bees make little holes in the wood, so we spray in there as well. They are gone later that day (dead on the ground), and I swear it helps keep the mosquitoes away.

If you are planning to make an offer on the house, I'm sure it's conditional on a home inspection. Talk to your realtor about your concerns, and I'm sure he/she can write it into your offer so that you have the matter taken care of before you close on the house.

A professional really should be able to talk to you about exactly which type of bee/wasp/hornet it is, how to deal with it and how to protect the house in the future.

Good luck!

Get an exterminator.

Are you sure they're honey bees? They could be yellow jackets or bumble bees or something else. In any case, get a beekeeper to come by to assess the situation. You don't want to do anything yourself if you end up buying that house.

I agree with Kristina M. Look for a beekeeper in your area. Here they will come out for free and help you get rid of the bees because they want them. Win Win as if they are honey bees their populations are declining. Bees are very important, if they go we are in deep trouble as they not only give us honey and polinate flowers but polinate our fruits and vegatables too. No bees=no food.

LOL, one mans pest is another mans pet. I'm planning on making "bee boxes" to attract bees to our yard next year.
http://familyfun.go.com/crafts/bee-house-876193/

You want to call a local bee-keeper to ask about this. He/she should know the laws in your area about this sort of thing. Where I live, honeybees can be transferred to another environment.

But if I were buying that house, I would insist on a VERY thorough professional inspection before signing a thing. If the seller is already sharing the house with bees, I would certainly wonder what else, known or unknown, is sharing the same premises.

If they are honey bee's I don't think you can do anything with them. Aren't they endangered? I would check on this first before someone gets a huge fine. My friend had them in her walls and had to call a professional to move them because they could not be killed.

Find a bee keeper. Honey bees are endangered and should NEVER be sprayed. Without honey bees we have no crops. They likely came back because they had a hive previously there if you don't seal the entrance to the hive ( where ever they came in ) they will come back , they can smell where they were before .