Can the FBI search a house that isn t owned by anyone without a warrant?

No, they still need a warrant, because of course someone owns it. A bank is an owner.

They can search without a warrant, if it is an emergency. Like if they are on a hot chase, and the evidence is going to disappear very soon. Or if there is a public danger -- the house is going to blow up and they need to find the bomb.

The house IS owned by somebody .. the bank.

The FBI can search when they want, where they want, with or without a warrant.

Whether they can use it in court is another question.

Every house is owned by someone, even if it's the bank. Even if the bank doesn't realize they own it, it's still owned by someone. In any event, a search warrant would still be needed.

All houses are owned by someone. If it's owned by the bank, banks are corporations and the same rules apply to corporations.

To search a house the police need either a search warrant, or the owners permission. An interesting thing about permission. It can be given before or after the fact. So if there's some type of emergency, the police can enter knowing that the bank will give permission.

If the property is owned by anyone, then the authorities cannot enter it without a warrant (except in certain emergency circumstances, such as if they believe on reasonable grounds that someone inside is in immediate danger).

If it was owned by the bank, then the bank has the same rights as a person living in the house would- they would have a right to refuse the police or FBI entry unless they had a warrant. The police would need to give the warrant to the bank in this case.

If the house was literally owned by nobody at all, it would be a public place and hence anyone could enter freely at any time.

There would have to be an emergency to allow a search without a search warrant.

I don't think so

Every house is owned by someone.

To enter a private residence a warrant is required.