I have experienced this a lot with my husband, who is high-functioning yet unmedicated. He would sometimes go out late at night and not come home until later the next day. Several times he told me he drove to look at the bay and fell asleep in the car. Other times when he was on road trips by himself, his phone ran out of batteries and there was nowhere to charge it or (this happens very often with him) he dropped it and it broke so he couldn’t call.
Every time this has happened, I have worried and every time he tells me not to worry. So far, he has always been safe and eventually returned home. (Although now he has been homeless for a year so that’s another story.)
With your brother: Does he have a cell phone? If so, the police may be able to track it if he has it with him. Do you know the usual route he takes or places he likes to go? Perhaps you could ask anyone along the route if they have seen your brother.
I am hoping that he is like my husband and just wanted more time to himself. Being missing overnight is worrisome. You are doing the right thing. Let us know what’s going on.