When Carol and Therese arrive at the Iowa motel, the desk clerk offers them the Presidential Suite rather than the two separate rooms they asked for, claiming "the rate is attractive". This is the room where they consummate the relationship and the private detective tailing them hears everything because he's in the room next door. He could have simply asked for the room after noticing that the two woman checked into the other one. . .or he could have let the clerk in on his plan and told her to offer the room to them.
In one of the shared hotel rooms prior to the hotel they sleep together in the camera pans from a blank wall to the two of them flirting. On second viewing it's clear the camera is focusing on the listening location of the private detective.
