This may already have been answered, but I just today listened to the radio interview of MR and Trishia was asking about the "pipe" listed in the Search Warrant.
MR described LE as swabbing the "plastic box in the wall" behind the washing machine. The technical name for this - in the world of residential plumbing - is a "stand pipe". This is actually a built-in box, recessed in the wall right above the washing machine, that has the hot and cold water spigots and also a drain pipe for the washing machine to drain into.
Many of us have the hot and cold spigot outlets to use for the washing machine hookups located on the pipes themselves that supply hot and cold water to a laundry sink, located next to a washing machine. The drain from the washer is a black hose with a hooked curve at the top, which hooks over the laundry tub, allowing the washing machine water to drain into the laundry tub.
In homes where the laundry area is located in a finished room, there is often a stand pipe. This has separate hot and cold water pipes (hidden inside the walls)and spigots for the washer water supply lines and a separate drain line for the washing machine drain hose to hook into. The spigots and the drain line are located within a recessed plastic box that has a plastic trim all around the edges.
This is what MR was describing. That LE removed the plastic trim, swabbed it, and failed to reinstall it.