The Chicken in a pot badge is for "Using 100 hiding spots "
Any place that you can interact with to "hide" counts towards this (under beds, in cupboards, In bins and inside closets). Each hiding spot only counts once per trial, so try mixing it up. The most efficient hiding spots are lockers, as each separate unit counts towards this.
Also two handy things to note:
1. The in-game badge menu tracks your progress.
2. Leaving a trial after gaining progress still counts towards it.
Video guide for reference: