Is there a squirrel in my house?

Okay, here's what's troubling me. Two days ago, I found a cracked open shell of an acorn on the carpet over near a wall. Is there any other explanation than that there is or was a squirrel in my house? I can't think of one, and I'm mildly terrified about the whole thing.

People are saying it's a big year for acorns, and you know I've been complaining about the noise they make hitting the roof at night. But that doesn't explain the broken shell getting into the house. And it wasn't just a crushed bit that could have come in on a shoe. It was a big, curved shell part, and, besides, we take our shoes off when we come in.

MORE: In the comments, people are giving me advice on how to trap a squirrel, but if there is an animal in my house, I'm calling the professional who solved the bats-in-the-attic problem a few years ago. My only question is, do I have a problem, not what to do about it.

So, is there a squirrel in my house? Why would a squirrel bring an acorn into my house and then eat it? The squirrels in my yard are running all over the place and burying lots of acorns. Why would one suddenly think eating one inside is a good idea?

Is it kind of like they way we humans go on picnics? We usually eat indoors, and then, on a lark, we say, why not eat outside for a change? So it's like a squirrel reverse-picnic. But you know I have trouble understanding even why humans picnic. Who am I to fathom the mind of the squirrel.