It is a bit of a mystery. Mice can squeeze through small cracks, but another question is how it climbed up. It could have climbed up the tires, but then what? Everything is slippery. And it wasn't clever enough to get back out, apparently.
Related mouse story:
I thought I heard a mouse in the garage. I started looking around for the mouse traps, and when I passed the empty plastic trash can, I looked down and there he was. He must have climbed up on something and jumped in. I was in a charitable mood and released him about 50 yards from the house
The next day, I heard the noise again, and when I looked, the mouse was in the trash can again—it must have been the same guy. He wasn't a good learner. I dispatched him humanely.