The hidden renders are unlocked by clicking on an object in a particular scene, for example: when Riley pins MC to the wall when they first meet, there's a box of cookies that disappear once clicked on.
The problem is that the clues are so vague it makes it almost impossible to find them. I've only found 2. The one previously mentioned and one where there is a dildo (I think) on the floor during one of the yoga scenes, (2nd?)