What's seen in that photo looks to me like
this kind of concrete block. Casey may have stepped on them when she took Caylee's body into the woods, but I don't think they are what Dominic Casey was looking for.
I think Miss Totmom revealed to someone that she'd placed 3 rocks or paver stones as a location marker. She either wasn't specific enough to pinpoint the exact location, they were underwater at the time, or the PI isn't good with directions (and was on the phone, trying to get a little verbal GPS positioning assist from his data source).
My guess is that she may have been a little vague in describing where precisely to look (
past the 2nd palmetto in a gully behind a big bush wouldn't help much), and what PI Casey was looking for were the three rocks - thus his foray into some abandoned backyard-looking place that happened to have three rocks in proximity.
It will be most interesting to find out who he was talking to on the phone during his woodsy strolls.