Whilst I have no doubt at all that this happened and completely agree it’s creepy af, I highly doubt that the app was somehow ‘listening’ to you.
More likely is that your partner has clicked like on a product tagged with the brand or browsed the brands website, or even stopped scrolling past an ad on Facebook for a few seconds and then continued without clicking (or a place that has some Facebook trackers). They can measure pointer movements, and placements, scrolling speeds and all that type of data. These things can be done almost passively without any real awareness of what your doing.
This was likely coupled with some sort of location tracking. Unless you (and she) are extremely vigilant it’s easy to have over looked sharing some sort of location data once, perhaps pointing to the journey. Perhaps it was even browsing the web or Facebook app from the location you ended up at, coupled with the data they hold on you knowing where you normally live, it’s not hard for advanced algorithms to work this stuff out. You took an a to b route, there are only certain main roads you would have taken I assume.
Just as freaky, but I’m certain these apps don’t ‘listen’ to you passively. On an iPhone certainly. That doesn’t make it any less creepy. In fact in my opinion it’s substantially more creepy.