I hit Cunningham Lake again this afternoon, with better results.
I arrived at Entrance 3, off 96th Street, at about 2:20 p.m. I scoped the lake from the fishing area, but didn't find any loons. I then walked out to the eastern point, where I found a Pacific Loon side-by-side with two Commons, about halfway across the lake.
After I'd watched them for a while, I started writing up notes; when I looked again, the Commons had disappeared, and the Pacific was swimming downstream, where I started losing it behind shrubs and trees on my shore. I went back to the fishing area, hoping that the loon would now be visible from there, but couldn't find it.
A Great Egret, presumably the one I saw yesterday, was also present, standing at the water's edge on the eastern shore. I also saw a single Bonaparte's Gull over the lake.