Therm-Optics
In the first movie they need special clothes and a veil for the face, right ? By the SAC they're running around going invisible pretty much when they want to.
Sometimes there's a blur sometimes it's perfect invisibility, which I assume is for the TV audience.
When Batou finds that sniper's satellite link (in SSS) and Saitou zooms in on him he can more or less see him, so Therm-Optics is kinda hit & miss ? or like Trek effects: "works as needed, when needed" ?!