I realize that I'm not revealing any top secret information here, but oftentimes I see people on the ice (and fishing open water) who give up just before the crappie become active.
I've fished a few lakes that offer good crappie fishing during daylight hours, but more often it appears that a lake or pond is barren of fish until the sun drops to just above the horizon. Then the Vexilar shows anywhere from a few fish to several feet of varying shades of yellow-green-red, indicating stacked fish.
And many fishermen who've been on the lake since sunrise with little or no success pack up and head for the bar/restaurant/home between 3 and 4 p.m. -- just before the sniffers turn into chasers and active biters as the sun drops.