I have a theory that Chase never even told Cathy the McStays were missing until after that first weekend. Based on him being the one who murdered them, per the evidence, I don't think he would draw attention to the fact of their disappearance while he's still got the bodies, is then out burying them and moving their Trooper to the border.
So when he's calling her during his interview with detectives on Feb 17th, asking what they were doing on Monday 8th, I don't think Cathy even realizes then that it was the Monday after the disappearance or what date the McStays actually disappeared.
There's something about the lateness of them discussing that he needs to speak to police even though he's got warrants, because this could be a life or death situation, and so when she's going back over her memories of Chase not answering Joey's call she could be mixing in information from Mikey (who has Joey's bill and the 8.28 call), as well as the Friday before the 4th where Chase didn't pick up Joey's call.
So I don't think she had a reference point for when they disappeared from the beginning. If they had have questioned her with dates and times closer to the time it happened I think she would have remembered Chase being absent on the 6th when he was in the desert, and maybe she would have clocked other things around that time, like his 20 odd calls to her that lunch time and the 5 calls she made to his voicemail that evening.
JMO