Dude im with faye on this one.
you show up as shes ready to get off. Shes probably drained from dealing with the crowds, mind moving towards home.... It wasnt personal. Im sure it would have been affirming for you if she would have smiled and engaged you in conversation... But since she didnt you could have taken the initive. You may have gone home WITH her instead of watching her her walk away.
Dont take it personal. Walk back in. Get her attention, some idle chit chat, 'hey looked like a busy night xyz day, didnt get a chance to say hi before you went home'
her: oh yes it was tough
day blah blah blah....
You: listening, noding.....
Her: blah blah finishing up....
you: wow that was busy (or tough or whatever fits) but i did miss having a chance to talk to you *you give a small smile* how about when you get off tonight we grab a bite to eat and talk then?
Have a nearby place in mind for when she says yes. If she declines.... Smile and say no worries, maybe another time :)