She brought up the supposed trip to Castle Rock for some reason, likely because she was there, and knew LE knew she was there.
I see a few possible theories as to where Gannon's body is.
She put him in a dumpster, and he's now in a landfill somewhere. I think this is highly likely, it's quick, and easy.
She put him in a secluded, wooded area, and covered him up with rocks, brush, etc. This is a lesser likelihood for me as it would not be the most assured in terms of Gannon's disappearance, and with drive time and/or work involved, I think in 'freaked out' haste, it would be too time consuming.
She drove to a spot and threw him off a cliff. Google Earth is a wonderful tool for use in this case, it really gives one a good look at terrain in the area. There are lots of places to go up in the mountains on back country roads, however I see no evidence of her having traveled in any of those directions, the exception being the area to the west of the search area on the 'S' turn on 105.
She dismembered him and placed his body parts in several spots. I find this unlikely as it would be too time consuming, messy, and leave way too much evidence.
So, I go first and foremost with dumpster. Put his body directly in a dumpster on the day she murdered him, OR (more likely in my belief based on the charges in this case) retrieve his body from where she first left it, put it in a heavy duty trash bag, OR some sports bag OR a suitcase of some kind, and throw it in a dumpster. From all observation on my part, she'd be able to do this without much difficulty. It is the quickest, easiest, least evidence producing, method of making her problem disappear.
Second to that, the cliff and/or wooded area with a quick throw over the side, or a covering up of his body, Somewhere west of 105.
