It seems to me that recognizing members of the crew would happen in three cases: 1) You coincidentally spot a crew member on another flight later in the day or the week, 2) You're flying a route route repeatedly, especially if it's a long-haul where it's more likely the same team doing it over and over, or 3) You have an exceptionally strong memory for faces.

I recognized an FA in case #1 once. It was a regional airline (I forget which) and she was on BOS-YYC one day and YOW-IAD two days later. I'm not a #3 type of person so that was the one time #1 happened for me.

When I was flying SFO-ICN and SFO-NRT pretty regularly a few years there was only once I recognized members of the same crew from an earlier flight. That, despite me making an effort to chat up the crew (passes time if nothing else) especially when upgraded to biz class.