Annie's Escape

After the Scouting Regiment captured the Crystalized Leonhardt,
They watched over her for years, waiting for her to break free and question her.
Then so, one day she broke free, and left without a trace.
Levi, sending out a Search Party to find the blonde; he sends out two groups to check all around.
First group is Levi, Eren and Mikasa,
Second is Erwin, Hanji and Armin.
Group one finds an abandoned tunnel and suggest to search there, Mikasa, slightly anxious herself decides to check out the tunnel and leave them to search elsewhere, in case Annie was a actually hiding there.
Why would Mikasa run them off?
Does she have a secret of her own to reveal?
Or just a selfish identity waiting to be revealed...?