4 people would be Faith, her husband, Butch (who was in his bus doing paperwork outside his house which wasn't far from the scene) and Butch's wife who i was imagining he'd tell but disregard her if you want, it's multiple people anyway. John Marrotte too if you believe his statement about Faith and her husband going over to the accident which would mean he was watching. Maura would have to go past all of their houses to get to the one you're talking about so i think several of them would have seen her going to the house, Faith, her husband, Butch and John at least. Even if her husband didn't see it she likely would have told him as we know they discussed the scene from the woman/man mix up.
I guess, it'd be odd that LE didn't find anything there though if she had been buried there, no DNA or anything as they carried out a proper search there from what i read and how long they were in the house. Those dogs are awesome but they're also fallible, so are LE of course but i'd personally bet on LE over the dogs. They said they found absolutely nothing and ruled it out. Especially adding in that i believe the witness statements (while acknowledging that they're flawed like any eyewitness testimony) and think they'd have seen something. There's definitely weirdness connected to that house though and CM so i understand people still being suspicious.