The Crack

(To start off with I just want to say that this is all true, so unfortunately there isn't a very satisfying conclusion. That's how real life is sometimes.)

         I think I heard it subconsciously a few times before I really noticed it. At the top of the stairs, just before I went down - a faint rustle; audible, but not something that would grab your attention. But about a month ago, I heard it louder, and it gave me pause. It was more than just some random noise - it was deliberate, the kind of scrabbling noise made by an animal. And it was coming from the crack in the post at the top of the stairs.
         I guess our house's layout is just a little unusual. A big part of the second story is a balcony-like area, that is, a large room with no wall on one side - just a railing overlooking the living room below. The stairs up connect directly to this space, so the stairs only have a wall on one side. On the other side is a railing, which meets up with the balcony railing at a right angle when it reaches the top. There's a post there. Like all the other posts in the house, it's made of solid, unvarnished wood (sort of a 'rustic' thing, I guess). And like the other posts, it's got some fairly large cracks in it, one of the largest (and deepest) being directly above the stair railing.
(These posts are big and sturdy, so the cracks aren't a structural risk. I'd also like to stress that these posts are solid wood, so there are no pipes or cables running through them that could be making any noise)
         Something definitely was, and perhaps still is, living inside that crack. The noise was unmistakable, even if I didn't know exactly what was making it. I leaned in closer, peering into the split in the wood. I couldn't see anything, but I heard it again. This was definitely a noise I had heard a few times before, but ignored until now. And for the next couple of days I heard it every time I went down the stairs. I never saw it though, not even once. It seemed that whatever it was would always retreat deep into the crack when it heard me coming. The crack was wide enough to see in a little bit, but it was no good - whatever it was seemed to be just beyond sight. Sometimes I felt intrepid enough to knock on the post, and my knocks would be answered by more rustling, as of something inside, squirming fitfully. I thought about shining a flashlight into the crack, but for whatever reason I never got around to it. Who knows, it might not have even worked anyway.
         For a few days the mystery creature was always at the back of my mind. What could it even be? It would have to be fairly small, I thought, to fit in such a small space. But at the same time, the noise seemed to be coming from something substantial. It was most likely a bug of some kind, but it seemed that it would have to be a somewhat sizable one if that were true. There wasn't much else  that could be small enough - perhaps a small lizard? I had found lizards in the house before, but it was hard to say whether one could fit in there. As time went on, it occurred to me that this situation seemed like the perfect beginning to a scary story. I idly wondered whether my parents would come home one day to find me missing, with maybe just a mysterious smear of blood around that crack. It was more of a (darkly) humorous notion than a really unsettling one, though - I can't say that I was ever actually worried. If anything, the mystery was kind of exciting.
Besides, it was probably just some bug. Our house is always full of bugs.
         A few days ago the noises stopped. The first day, I didn't think anything of it, but two days without hearing anything seemed unusual. Tapping and knocking on the post, even on the very edge of the crack, got nothing. It seemed that I might never solve the mystery. As of now, as I sit here writing this, it's been more than a week since I last heard anything. Despite the implications of vermin in my house, I admit I sort of miss it now. Still, there's one thought that nags at me when I'm trying to sleep at night. I can't help but wonder, if it's not in the crack anymore,
where in my house might it be hiding now?