Happened to me aswell. Went three times through the whole level to see if I somehow missed an item but I picked em all up. Still the game tells me 37 of 38.
Mummy room at the bottom floor after you defeat the 2 Mummies at the bottom the music goes off when you battle them after that there is a switch in that same room take 2 or 3 jumps to get to.
I guess you mean the bridge for scarab as for the other three bridges you can see it in this video. 04:27, 11:55, 13:04 For scarab just go straight forward at 14:41 and follow the way. Have to do few jumps and climbing. 15:14 the little hole under the blue is where you need to go. There is the switch.