What was in Bill's desk?

[nikvoodoo]

Could the other item possibly be the silenced pistol?

Only if you can figure out when he would have had time to put anything in Bill's desk after Pippin's assassination. Once Pippin was assassinated, Kalani was gone from the Tower...forever. He presumably never went back inside it's walls. He returned from LAX after inspecting the Jolly Green Giant to pick up Angel and Riley, returned in the Bell only to drop Angel off and take off to LAX to pick up Pegs for training. After that we know what his movements were because it was all covered pretty clearly by chapter 24.

We certainly know Kalani isn't capable of climbing 10 some odd flights of stairs and still escape the Tower after shooting Pippin without being caught, so if he was the assassin he would have ditched the gun somewhere else in the building and not upstairs.