[Engineer laughs ruefully. He has nothing in his hand, and he's about to have nothing on his body, either. He sets down his whole lot of nothing as he continues, his mind still turning over his hypotheses in his head excitedly. The alcohol helps.]

I would've thought they somehow harnessed the heart implants, but those watches don't have any effect on the rest of us, so it's gotta be somethin' they put in you Spies only. That invisibility doesn't make you invulnerable, so it ain't like the Ubercharge, and I'd think if your Medic was the one who put it in that there'd be a lot more invisible Medics on the field.