Yup they are jumpers allot of Gobi's are I have a lid kind of once they have been in the tank for some time they seem to settle down on the jumping a bit. I leave the lid off during the day so the lights are not hindered in any way and more light gets to the bottom of the tank. He may have dug a hole and the rock has shifted and he is dead under it. They tend to do this also digging holes under large rocks is a bad habit of theirs and it can be dangerous.

Bill