That could be through a fire caused by your negligence or something as simple as closing the door behind you when you enter the building and accidentally catching the neighbor's hand in it. (effectivecoverage.com)
When he goes in after the toy, you can quietly close the door behind him. (raisingspot.com)
Make sure also to be careful when closing doors behind you that the puppy does not get stuck since they have a tendency to follow their owners everywhere. (k9puppydogs.com)