In this one, Justin and Brian are at a home improvement store (I believe Home Depot or something). Justin is looking at paint samples, while Brian is looking for something to preserve Gus's footprint on his loft floor. Molly and Craig are also at the store and Molly screams Justin names upon spotting him.
I was thinking it was one of galeandrandy's fics, but no such luck.
Spring Fever by Rizabeau sequel to New Year
Link in the comments