I think a stray cat went underneath my building. It must have gotten in from an uncovered hole on the outside. First, I heard it through the chimney and was wondering where exactly it was. Days later, I heard it through the heating vent while talking on the phone. All of a sudden, I hear a loud meow and I went ahhhhh! My friend said, "What happened?" I told my friend that I think I heard a cat in the vent. They said, "You sure it wasn't a mouse?" I said 'I'm sure. It went meow.' Then we continued our conversation.
I haven't heard it recently and so it probably left.