Do you have a moist hide in the tank? And right before they shed, they turn really grey, basically to where you cant see the their colors. Once, my male Pictus gecko was about to shed, super gray one night, and he was in the moist hide when I went to bed, and the next morning he had already shed and ate any evidence. But they shed every few weeks. But he could have shed without you even knowing it.
Moist hides are super important for helping them shed. They provide moisture to loosen the skin. You can cut a hole in a Tupper ware and use damp paper towels or buy moss and mist is regularly. Just do it