The rebreather should take more than one scan, and should probably only be found in places where your oxygen efficiency is already decreased. Also, I think it would be a very neat gameplay progression idea for the rebreather to require an upgrade. At around 700 meters or so, the rebreather could fail and your oxygen efficiency would begin to decrease again, requiring the player to locate an “improved rebreather” scannable or databox, and craft it at a modification station.