Even though he couldn't comment when asked about how long her body had been there, it sounds like they are assuming it has been there since Saturday or Sunday if they are asking about people who were muddy over the weekend. So it doesn't sound like they think she was held alive somewhere for any length of time after she disappeared, if I am reading between the lines correctly?
I think so too. Another possibility is that they have evidence of premeditation? Maybe the person went out to scout a dumping location on Saturday?
Eta- no I take back the premeditated thing. It doesn't make sense.