It feels like they already knew exactly what they wanted to do at the scene... no matter what they may have found.
My guess is they wanted to create an impression of something that wasn’t. You know create a frame up situation. These people are power hungry MOFOs who will stoop to any level because they know no bounds of decency. They are true criminals, and dangerous criminals at that.
They had a few extras left over in storage, right next to the hand-written "Baby Milk Factory" signs.