It depends where you are walking if you want a realistic answer. Lots of people walk around denser areas at night with commercial stores/nightlife and residential areas near transportation. Walking around the suburbs when it’s 12am and there is nothing open is probably going to raise suspicions because it’s uncommon and there’s not many places to walk home from other than a friend’s house. (Not to mention it’s dangerous with all the people speeding/not paying attention to the roads when less cars out at night). IMO, I would say the more empty an area is the more suspicious it would be to walk around at night. There’s legitimate reasons to walk around at night like going for a stroll or walking a dog, but for the most part police would stop you to let you know “you do know this is risky?” and let you go on your way as long as you don’t seem like you are planning some criminal activity or acting erratically. Plus, it lessens their chances of having to go help someone out of trouble if they just advise that person reevaluate their decision.