The bomb did not go off in the arena, it went off in a vendor area before the ticket check, in a public space.
Her concern about someone acting oddly is irrelevant because the actual bomber was a male with no ticket who detonated in an area before any security. Much of her concern is after-the-fact.
Good post. People are confused about the sequence of events. The bombing happened OUTSIDE the arena, when people were LEAVING.
Early reporting included a second explosion. People there reported same.
“The stewards were trying to calm people down but everyone was running in a stampede,” recalled James, who made his way out of the arena through another exit. James said he believed he heard a second explosion as they were running out.”
http://wtvr.com/2017/05/22/manchester-blast-witness-bodies-scattered-about-everywhere/
Moreover, the public area was right next to Victoria station, which would have been an equally target rich location had the killer not gotten into the entry area.