What does it mean that people in the police station wear casual clothes to unlock the lock at night?

It is possible that he is not from the police station, but a thief. It is also possible that they want the owner of the house to help solve some cases, or there are clues in this house.

I don't think normal people will break into other people's homes in the middle of the night unless they are thieves.

If they were plainclothes policemen at the police station, they wouldn't have unlocked the house.

I hope it helps you!