Right. The time of day is the clue. When I've reared butterflies from eggs or larvae, they've always eclosed in late morning. Then they rest for a few hours before they're ready to fly. However, my experience is mostly with anise swallowtails, a few times with other species, but no pearl crescents. I'd guess that your butterfly had spent the night where you found it.
Thanks for your input OhLook. The main thing I like about getting out before daylight, is that most of the things I find are still sleeping, which allows me to get in as close as I like, usually 8~15"without having to worry about them taking flight. Although sometimes a dragonfly will wake up and take to the air. Its also the coolest time of day, so sometimes it's too cool for even some night fliers like moths to take off without warming up first. But this fluttering before takeoff even offers an opportunity to get the underwing that otherwise I would never see unless I captured and stuck a pin through them, which isn't my thing.