While anything's possible, it would be pretty shocking if Facebook figured out how to, on iOS (I can't speak for Android):

* Record audio if the app's killed completely

* Record audio with microphone permission disabled

* etc.

All of which have been claimed.

The far more likely scenario is that we're much more predictable than we think. Remember the target controversy when they figured out a teenage girl was pregnant before she had told her parents [1]? That happened in 2012. Imagine how much more advanced things have become since then.

1: https://www.forbes.com/sites/kashmirhill/2012/02/16/how-targ...