What's the faintest thing you've ever seen with your own eyes?

I mean that in the astronomical sense — the dimmest star, the barest smear of comet, the last trace of a meteor. But I'm also thinking about the dimmer things: a memory you almost can't retrieve, a light in a window at 3am, the moment you catch someone trying not to feel something. What's the faintest thing you've held long enough to actually see?