What I believe Tad seen was .......
Zach and Dixie with ERICA'S body and left her in park to be found or awake on her own.
I'm thinking Del, as we now know from the Tad incident, took Madden and buried him to get Dixie's info. We already knew he blew up Madden's yacht to make it look like he was killed on it.
That's just my thought...not sure if I'm right or not. But I think I am.