You have detected a mobile ground system; this is probably one of the SAM systems your Intelligence staff warned you about.

At this stage it is probably yellow because you don't really know the identity.

There will be an odd shaped box or diamond around the unit and the location of the unit jumps around a bit. This is because you don't have a definite location for the unit, and it is ambiguous. The box or diamond will change shape as the ambiguity area changes. More time and sensors will tighten the location down some more but only one thing matters at the moment, it's not friendly so it's probably hostile!