I was in line at the RPGA Dungeon Delve when the alarms went off and we were told by loudspeaker to evacuate.
My group and I figured it'd be awhile once we saw the two big trucks pull up, but from what I heard today at Gen Con, it only lasted about 10 minutes.
Thanks to the telephone game, I'm still not 100% sure what happened, as I've heard anything from small fire, to bomb threat, to some smoke. Either way, we watched local news all night on Saturday and nothing was mentioned, so it must not have been major.