Mouse in the House

Josh found this on our kitchen floor last night. It was already dead. Here are my questions.... How did this mouse get into my house and how did it die?? I'm glad we didn't have to chase him around and set up traps to catch him, but why would it choose my kitchen floor as the place to spend his last moments of life?

One theory is that because the lawns were sprayed yesterday, he got poisoned. That still leaves the question of how did the mouse get inside! If one mouse can get in that means another can get in, so even though this one is gone, what's to say that his friends won't find a way in too! I hope I never have to find out.