Since they're searching near Farmington it's likely they have reason to believe that he brought the body back up there (maybe some evidence in the 80MS house, like her blood in a drain, etc?)
But, since they're back to looking in water, if there's reason to believe that the body could be in the vicinity of NC, I have always thought that Browns Reservoir along Silver Spring Rd/East St would be a logical place. I don't think it's fenced, it's quite remote, it's along the way between Welles Lane and Pound Ridge, and it would be easy to connect back onto Rt 33 to 7 or up to 84 to get back to Farmington pretty quick.
It's also "just over the CT/NY line".
I know this to be a place where teens can go to do nefarious teen things, private and out of the way (not like murder, mind you, stuff like many people do when teenaged).
Not sure how full the reservoir is at this time of year, but easy access to water and not likely to be seen back there.
Of course, there are literally hundreds of potential spots between NC and Farmington. I'm sure LE has information that's driving them to strategic possibilities.