First off. NEVER leave them unattended while they're out. Never. However safe you think they are, they aren't. They can get to places you wouldn't even dream off. Personally I only let mine out for an hour max at a time, but everyone is different. Too much exercise is bad for a chin because they over tire themselves and they can actually die of this.
Now onto the important part, finding you chin.
Try to lure him out with treats, especially raisins. If all else fails leave piles around the rooms to tempt him out. Check behind anything no matter if you think the gap is too small. They can squeeze up behind chests of draws and get into the draws. Seriously check everything, under, behind over. Inside cushions and couches. And use treats.
And... make sure you do not open any windows, because they can jump very high, some up to six feet so no matter how secure you think they are, they aren't.
I wish you the best of luck trying to find him, so chins just like to hide. and just never leave them unattended again. Also I hope you're rabbit's okay and please don't be offended by what I have said.