Bit late to this thread but I've been doing some digging since seeing something very similar over Somerset about three weeks back. Not Phoenix obviously but the shape matches almost exactly what I captured on my dashcam - silent, slow moving triangle with a light at each corner and a dimmer central glow underneath.
My question for the OP is did yours make any sound at all, even a low hum? And were the corner lights steady or pulsing? The reason I ask is that I've been comparing notes with a few people from the Bristol UFO group and we're trying to work out if these sightings are connected or just similar craft operating in different regions at the same time which honestly would be just as interesting if not more so.
Also what direction was it heading when it disappeared from view? Did it just vanish or did it genuinely move out of sight?