It would be the concrete pad on which the garage stood misty, not a panel.
You say dogs have been know to fail to alert to buried bodies wrapped in plastic sheeting. To me you appear to be implying a general statement that all dogs would not alert. The relevant question IMO is has Eddie ever failed to alert to a buried body wrapped in plastic sheeting. I think the answer to that is no. IMO your statement on this has no validity in the McCann case and could have no validity anywhere else without a cite.
Please think carefully about the implication of the following statement for which a cite is provided.
The cadaver dog was apparently unable to make an alert when the item for inspection was wrapped in brown paper and only did so when the paper wrapping was slashed on request.
SnipHe said they then went to a Detroit Police Department evidence room, where investigators wrapped Bianca's car seat in brown paper and hid it in an office within a number of rooms.
Grime said there was no response during the first search, when the car seat was sealed inside the brown paper. He then asked officers to put a slit in the paper and move the car seat to another room.
"The second time, when the dog got close to the package, he put his nose in the package and gave a positive response," Grime said.
http://eddieandkeela.blogspot.com/2012/10/expert-testifies-that-cadaver-dog-gave.html