the markers are in slightly different locations on each range, but you will find one by the trackpad adjustment screw at the bottom, one by the top of the trackpad in the middle, one on the corner of the backlight sheet for the keyboard (by the end of the subwoofer) and one under the corner of the optical/logic dividing bar.
I often use them when completing a liquid spill repair, as it can indicate how far spread the liquid was initially. They will be white when they are untouched, but will go a pinky/red when touched by liquid.
As i do repairs and am not associated with apple, i dont use them to identify whether it will be covered under warranty, it just indicates how deep of a clean needs to be done before rebuilding it around a logic board...
i hope this helps