In the last part of the level Outskirts, you make your way through the enclosed highway. Partway through, a Scarab blasts a hole in the roof. The glitch is that you can climb on the debris and jump onto the roof. Though Grenade Jumping helps, it is not neccessary. If you do it fast, you can see the Scarab walking away and then disappearing.
If you can fly through the level and get to the part where the scarab blows a hole in the ceiling you will also notice very faintly a scarab gun in the blast. No one yet has been able to obtain it and prove. Maybe you could be.