TheInformation's article includes a bit more info and this tidbit:

> As most owners probably have experienced, the microphones can often be triggered inadvertently. And those errant recordings, like ambient sound or partial conversations, are sent to Amazon’s servers just like any other.

When I use my Echo in my apartment alone I don't have a lot of false positives on the wake-word detector. This past week I've been in my family's home and very frequently when more than one person is talking Alexa has butted in (and tried its hardest to understand what we're saying, listening for the maximum time it can).

I think the hope is that if some ruckus was happening in the home, the Echo might've accidentally woken up at some point and recorded a small bit they can use to help piece together the crime.