In modern times A/B/O characteristics are considered genetic relics. Only a small part of the population has them, they don't affect society at large too much, relationships aren't restricted, it's mostly a non-issue. But Andy's an alpha from a time where they were everywhere: old-school, full strength omega catnip. Over the years as A/Os faded out and customs changed she got good at ignoring whatever hints were left unless some enterprising omega was in heat or bold enough to hit on her. Joe and Nicky were both betas, and sure Booker smelled a little different but it was a different time and place and she assumed he was too. Nile, a modern alpha, had barely met them and wasn't about to stick her nose in, but after Booker's been exiled she mentions that it just felt wrong to send an omega away like that on their own. . .and it turns out the others never knew. Booker never said, and spent enough time on his own that he was never in heat around Andy for her to tell.

I'm not picky about pairings—Andy/Booker, Nile/Booker, anything else you might think of—so long as it's not super angsty or noncon (. . .so probably no alpha!Quynh/Booker, but other than that).