If the dog was let out, it would eventually come back and start barking outside causing lots of noise to other neighbors, I would think.
I'm the owner of a professional, full-service pet care business for over 15 years. Fortunately, I've never had a customer's dog get out of the house while in my care or away from me on a walk. But I have heard customer's tell me about such instances with their dogs. My general observation is it's totally dependent on the dog, it's training, and response to commands whether it runs off in a flash or if it runs around the property and then comes back to their home/owner. Dogs can travel very quickly if they are on the run, but most will stay in close proximity to where they 'lost' their owner. Hard to say what Murphy might have done if let out of the house without a leash.
I know I have a number of times seen what I could tell was a 'dog on the go' and stopped my car to try and secure the dog. Sometimes successful, sometimes not. If I have not been able to find the owners I have taken the dog to the local Vet that know my business to have it scanned for a micro-chip. If no success, and no idea who the owner is, I call our local animal control officer, and if that is not an option then the police to come and secure the dog.
If Murphy did get out and was wandering around late at night, wee hours of the morning, perhaps someone saw him, was able to secure him and called the police to come get him. Or did the person find Murphy, see the owners phone number and address on his dog tags, tried to call the phone number and left a message for the owner to call them regarding Murphy, and took Murphy home with them to wait for the owner to call them. And maybe waited Sunday morning for a call from the owner, and finally late Sunday morning after no call from the dogs owner drove over to the address on collar tags (if it's on the tags), knocked on the ground floor door. Knocked and knocked and one of the downstairs surviving roommates heard the knock, went to the door to see who was there, and found Murphy with this person. They go upstairs to find Kaylee, the owner of Murphy, and then who knows what they encountered and what transpired at that point.
Total speculation on my part but not inconceivable.