Dark object sighted over Old Faithful

My friends and I were at West Yellowstone for the annual car show they had there the first weekend in August.
Sat.night after the dance six of us decided to drive up to Old Faithful and watch it erupt..We left W.Yellowstone and drove up in two cars around 10:30pm It's about a 1/2 drive to Old Faithful.
When we got there,there were about 20-30 other people there to watch the geyser,It was supposed to erupt around 11:45 so we had some time,We stood around and talked for a few minutes,
This guy I was standing next to noticed an object flying towards were we were all standing and told us to look,As we did he tried to look through a set of binoclars but couldn't see very well,He then handed them to me for a look,
What I observed was an dark object gliding through the air,it was low about 500 ft..We noticed what looked like a search light pointed to the rear of the object,as it passed overhead,the light turned to the front of the object,it made no sound as it passed,it looked like a round body with a long tail..like a helicopter..but with no sound..
(mysterious black helicopter,maybe?)
Witness indicates that date is approximate.  PD