"Ground. You know the way a stealth cloak can ripple? Easier to spot them against the skyline but Fallen climb like geckos."

And they also shield themselves from more wavelengths than a human - or Exo - optical range. The cloaks are imperfect, still: look very carefully, and among the broken outlines of the rocks and the odd energies of the Citadel Samus might spot a hunched outline. A spot where the infrared radiation from the sun-warmed stones bends in a suspicious way...