1) Aware of construction happening close to your place of employment (in any number of ways that doesn't require any super advanced knowledge)
2) Knows that people usually start to get fed up with construction noises after X days
3) Started showing you popular co-working spaces as a result
Or, as you noted, your co-workers started searching for co-working spaces and Facebook picks up on that and assumes something is happening in your office such that other people might also be interested in co-working spaces. Creepy, sure. But doesn't require clandestine recording and parsing conversations.
Or, even simpler than that: I see WeWork advertisements all the time despite never discussing them. It's not really that insane to think that places like WeWork might just be targeting your demographic and that's why you saw the advertisement.
Anyway point being, none of these explanations require Facebook to record you.
I think the example another person gave is almost just as creepy -- your friends influencing your own advertisements. That means that what your friends are searching for, private (embarrassing) things, could potentially leak over to you.
I'm pretty aware of the fact that if I go looking for socks, I'm going to start getting sock advertisements on almost every page I go with advertisements, on both Google and Facebook. I don't find that creepy, just dumb and ineffective because it usually starts well past the time I already bought the socks.