Help! Ball python got out!

[Snakeypoo]

So I come home from work today and I go to check on our BP and he's not in his terrarium. WHAT THE heck?! Either he learned to push it open without knocking the two heat lamps over, as they are over the lid (you have to remove them to even get the top off) OR my boyfriend left a crack in the top open. Whatever happened, our BP is now missing. I have flour down, and a mouse in a box and heat lamps and his water dish near-by. Our apartment is on the older side, so there are a few holes in the baseboard where the pipes for the heater goes into the wall. I have a feeling he's in there. Is there any way to get him out? I turned the heat down quite low, so it's not like the heater is even warm. Do snakes most of the time show up when they get out of their enclosure? Or should I just hope for the best, but not count on it? I'm just really sad and worried and I don't want to lose him forever :'( Any help, or positive thoughts would be fantastic! Thank you so much!