If they have positional data they could do a correlation of location speed, along time time of week. If they constantly see you going to the same places on the weekends, that might be a clue, especially if in close proximity. Especially if traveling at the exact same high speed in close proximity (Driving there). GPS, at the high range, can tell location within meters.

And none of this would have to happen while you were talking about cars, or anything else. Just enough times to make the correlation, and with data that could have been collected months to years ago. Google Now did that for my commute from work to home.

I think it is more likely Facebook is using location data to create edges on a shadow social network. We just bleed metadata.

My current suspicion is they think my phone is hers due to IP. She doesn't use FB mobile at all and I use ad-block + NoScript on my computer, but not on my phone.

I've picked two different, random topics (boats & umbrellas) to occasionally search for on my phone and computer respectively. So that should help me figure out what the source is.