As long as he is eating and his litterbox habits are the same, then it could be that something scared him. Does he have access to a window or is he closer to the kitchen? (as some scents while cooking can freak bunnies out). Also, being in a new apartment, even though he is used to it, a new noise could have made him more cautious one day. Something he could hear, but you couldn’t.
This happens to my bunny Aria. She knows every regular sound, scent and sight, but something new — something she spotted in the window, something “icky” she doesn’t like the scent of for dinner, or a vibration or noise she isn’t used to — she will go and hide for a long while in her hidey box. I figured out that the washer, when it was imbalanced, would shake (of course), but the vibration of that ended up freaking her out. She is not even next to the laundry room, It’s a room away, but it’s vibration was strong enough that she could feel something funky going on.
All okay with his diet and litter box habits? If so, just a bunny freak out that we humans couldn’t figure out! 

