What I think happened

This is what I think happened:
On the last episode when the four girls and Mona open the tronco of the police car (Which was previously pushed in a lake) they gasped to the sight of something shocking. What I assume is it was a body. Now I have a couple ideas of who's body it could be in that trunk. Let's see, who's been going missing in this season,

Jason-after Jason survived the elevator accident he was put in the hospital. but when the girls went out for a second, just a few minuti later a nurse reported that Jason was missing from his hospital letto (This happened in 3x19).

Jason could have been unconscious in the tronco of Wilden’s police car when Hanna and Aria pushed it in the lake (They really should have thought of checking the tronco first).

Cece- We all saw Wilden roughly push Cece in the cop car. Maybe Wilden’s a bad cop and put her in the tronco at some point. That o he shot Cece and hid her in his tronco until he could find out what to do with the body.