What was the cloud spotted near the western horizon August 31, 2004?

Around 9 p.m. on August 31, 2004, I saw a bright patch of light about half the size of the Moon near the western horizon. It moved slowly upward and fluctuated somewhat in brightness. Through an 8-inch Schmidt-Cassegrain telescope here in New Bloomfield, Pennsylvania, there were two points of light moving along with the cloud (see sketch). Within a half hour, as it neared the zenith, the cloud spread out and I lost track of it. I’d appreciate knowing what this strange phenomenon was.

Centaur 2nd-stage rocket

The Centaur-2A second stage of an Atlas IIA rocket.
NASA

We received a flurry of e-mails and phone calls about this cloud, which was widely seen over eastern North America. Online discussion forums for satellite enthusiasts quickly identified the cloud. It was the result of excess fuel being dumped from the spent Centaur upperstage rocket that had put a classified US reconnaissance satellite, NROL-1, into orbit earlier that day. From his backyard in Australia in October 1997, spacecraft expert Tony Beresford witnessed a similar cloud following the launch of the Cassini-Huygens spacecraft to Saturn.

— Dennis di Cicco