I know normal mexes' radii appear. But in this particular case, it was a cloaked mex near my defence line. I fired on the metal spot a few times, but it wasn't placed directly on the metal; the F4 map enabled me to find it, though, and force-fire a Gaat gun on it.
It's a problem in the underlying design, making it hard to fix, unless the code is made either 1) much slower 2) much more memory hungry or 3) the metalmap view is disabled totally.