We recently had a flooding in a detached building that has a half bath. We have a very small water heater attached to the faucet. The hose going to the hot water faucet is detached and that's apparently where the water came from. We have not had any slow leaks indicating the hose was loose. It is just off and flooded the whole building. Could a faulty pressure valve be the problem? We are totally puzzled at how it came off.

What kind of hose is it? Did the hose come with the faucet? Is the hose adequately rated for temperature/pressure? How was the hose attached to the hot water line?

A hose can slide off a barbed fitting without exhibiting any warning signs.

In addition to excess pressure, excess temperature or a poorly made joint could have also caused the failure.

good practice would be NOT to use a hose for pressurized water but to 'hard pipe' with actual metal tube

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Could a faulty pressure valve be the problem?

Yes, but there are plenty of other more likely reasons.

Unfortunately there is no way for anybody here to make conclusions without photos and more specifics about the type of hose and attachment.

Did you try pulling the T&P release?

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Links to the State of Wisconsin Plumbing Code:
[docs.legis.wisconsin.gov]
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I am not a plumber.
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It could just be a "cheap" hose.

If theres any chance of freezing, that would be a possibility also.