Comet-like object seen moving across the evening sky (but cometsdon't move a few degrees in a few minutes!)

Last evening I was in Vallejo, CA (38n,122W) at 1830 pst and observed, to my astonishment, a naked-eye comet-like object moving across the sky about 20deg S of Orion. It appeared to be a barely visible "nucleus" trailing a distinct triangular "tail" appearing about 1/2 degree (full moon sized)in diameter. I observed it (and confirmed it by asking a passer by if he could see it, and he said it appeared to be a "comet") for about 2-3 minutes before it moved east and faded from view--much like a satellite flying into the earth's shadow.

It was much dimmer than the ISS and really did resemble a "comet". I've observed many real comets in my days, so I'm no stranger to their appearance, but this appeared to be some orbiting object, perhaps venting propellant. Any ideas what it might be?