My dog has done this too. I figure if neighbor leaves her door cracked open to allow her cat to roam the hall, I don't feel too bad. Of course the dog is going to be curious about the cat and new smells.
That's puppies for you! I don't give them an ounce of "well behaved" trust until they are like 2 at the earliest. Until then, everything in life is fair game to them, Lol! How did she manage to run in there? Little stinker!