Tim Frazer of Pocatello, Idaho (fraztimo@nicoh.com) writes:
" At 19:58 Mountain Standard Time around 23 February 1997, I observed a small bright cloud halfway between Sirius and Procyon. Ten minutes earlier it had not been there. I was looking at the Orion Nebula. The object covered 4 x 2 degrees of the sky but there were no other clouds in the sky on this very clear night. It didn't move at all and I could see stars through it but they didn't twinkle and looked sharp. By 20:07 the cloud had vanished."
Because it was so soon after sunset, I would have to guess it was what is called a 'noctilucent cloud', at very high altitudes, illuminated by the sun. I originally thought you were talking about the Pleiades, but this 'cloudy' star cluster is about the Moon's size of 1/2 degree across or so and too small.