He is lovely
They do sometimes go and sit in the corner of the playpen - usually means they've had enough and want to go back to the cage. Do you have some hidey places and toys in the playpen - something they can go and sit in (mug on its side eg) or a tissue box.
He was probably a bit stunned when he got back in the cage and found things slightly changed.
It might have been better to leave his hoard alone as he's only just settling in. If it was pee'd on you need to remove/replace most of it but always best to leave some of the old hoard.
Is he not using his litter tray? Maybe move it to where he was peeing.