The Boomer is one of the cameos from the videogame Left 4 Dead that appears during the movie, along with the Charger, Hunter, Smoker, Tank, and the Witch.

The Boomer can be seen in a chamber two columns over from where Marty and Dana reside during the pan out shot that reveals the assortment of monsters during their elevator descent into the depths of The Facility.

Due to its abscence from the whiteboard there is presumably no item within the basement to summon it.

It was placed here because the directors originally planned to release Left 4 Dead exclusive DLC that featured the Cabin In The Woods cabin and facility. This idea was eventually scrapped when the company producing the DLC went bankrupt early in the process.