Well, several years later, I've found a couple more of these. One I found dead in the same parking lot (probably came to a light), and I just had one hit my kitchen window tonight, attracted to the light. I caught it because I recognized that it was a bigger thud than the Phyllophaga hitting the window.
So it seems they may come to lights in spring?

I've seen them late May to late August on Block Island. In June and July they're especially common. In fact, last week I found seven at the porch lights in one night.