How's this little snake get in my fireplace?

A harmless little garden snake -- maybe a foot foot and a half or so .,. thicker than a pencil for sure, probably because of last's month's all-you-can-eat cicada buffet. And it's a little suburban living room brick fireplace. Nothing grandiose. No wood in it for months. However, the fireplace is covered with a metal fireplace screen door and the holes in the mesh are way too small for something to fit thru. And the whole cover is flush with the brick... I don't see any obvious ways thru it. (Maybe where the bricks' cement mortar indents a little, but that's a very, very tiny sliver.) The snake was behind the screen, in the fireplace, trying to get out when the cat spotted it moving. We certainly would have spotted it if it had been in there for long. I'd hate to think the thing slithered thru an open door, down the hall, into the living room, thru the fireplace cover somehow, and into the fireplace. It's not like we wouldn't notice a snake visiting when we open the door to outside. We've had the air conditioning on lately due to the heat and humidity so we're not leaving doors /windows open. All the windows have screens. So that leaves... down the chimney? Who ever heard of snakes going down a chimney? (I have a spring loaded cap on it anyway and the damper is closed.) So that leaves... unfortunately... a crack in the fireplace somewhere? But I don't see any obvious cracks in the fireplace box. None. That snake was pretty fat relatively and a hole that size would be noticeable. But a hole in the foundation into the fireplace box seems the most likely, right? That would suck.