Couldn't have been Stealing Babies since they performed it at a few times on the Gravity tour.
If we're talking album material, it''s gotta be something from this list:
Dirty Walls, Is It Safe, Julia (they've soundchecked it though), Under Zenith, Happiness & the Fish, Blister, Lying Awake, Annie, Consequence of Laughing
Depending how you think about it, these may also be considered old songs that they haven't performed since he's been with the band.

Sell My Soul, Sorry (they soundchecked that one), Don't Stop, Refuge, Never Get Over You, Signs of Life