That's just how social expectations are. I recommend you go ask a handful of single men you know (i.e. coworkers) whether they'd consider a 1:1 outing with a woman to be a "date." I'm guessing most would say yes.
Exactly, they're coworkers where OP isn't certain if they were flirting or just being friendly, and other coworkers confirm she's flirting. To me that means:
they don't have a good enough relationship to tell the difference, so probably not "friends"
other coworkers don't seem to experience the same thing
claims to not have other friends (surely she'd mention a BF, no?)
So to me, that sounds very much like she's flirting, so it's totally understandable for OP to consider it a date.