FreeCampsites.net pointed me to Maple Hollow Campground, up a canyon from Holden, Utah. Even though it was technically still Labor Day weekend, I had the place to myself. Sweet.
After it got dark I noticed some fairy lights hung at a neighboring campsite. I assumed some very quiet campers had slipped in without me noticing. But come dawn there was no one there, only the lights, which were still on. Hmmm. Either the neighbors had slipped away just as quietly or they had been there some other day. Whichever, they forgot their lights.