I've been thinking about why DP chose that particular day to make Stacey 'disappear'. According to articles, Stacey had been conversing with Pam (I think) about possibly renting one of her properties.
IIRC, The place Stacey was going to help paint was one that had previously been painted for their brother, but because he went back to jail, he was no longer going to need it. They were preparing it back to it's normal state (white walls) to be leased again. Maybe DP felt that they were actually painting the apt/home for Stacey to move into. So, thinking another divorce was looming, he took care of it before it got to that point.
Just a theory.
This is a good possibility. On the other hand, I do believe that he either listened to her conversations, or overheard her talking to her family about leaving him. It's been widely reported that he stalked her. It's not out of the realm of possibility that he would listen to her phone conversations, or track her computer use. I also think she just may have come right out and told him that Sunday morning.