My guess would be that something happened to the tenant. If someone doesn't show up for work, or there's cause for concern, they usually send a fire engine and a police officer over to break into the home and check it out. (I dated a fire fighter, and he'd broken down several doors and a couple windows in similar cases.) They usually send fire because if there is a lock box on the building, they can open it, and they're usually trained paramedics, which police officers aren't, always.

That's in Massachusetts, anyway.

I hope your neighbor is okay though. Maybe it's something else. and she's okay. A gas leak?