As a cop, I was trained as a Rapiscan operator for the Commonwealth games in Glasgow.
Trying to identify an explosive device is incredibly difficult - it's based on organic or inorganic material. So everything is pretty much the same colour and density on the scan until you come to organic material such as an orange.
All explosive material is organic.
What you're then looking for, is the combination of inorganic leading in to organic. So for example, a dense inorganic signature (say a small square which could be batteries) with an inorganic trail leading from it (such as headphone wires) leading to the organic material (your orange for your snack on the flight). That all looks like a bomb and I'd stop it and get it searched, unless I was really certain in which case we run like fuck.
There are also some materials which basically cause black shapes and have 100% density, which would be an automatic search, such as a lead plate.
It's really incredibly difficult to identify a bomb in a carry-on, and it's about who the owner of the bag is that makes the difference. We use Behavioural Detection training to add up the chance of that bag being packed with an explosive device, otherwise there would be a lot more bags getting searched just to make sure - but then people would miss flights, queues would build, complaints would rack up, etc, etc.
Putting it in the hold is no different from carrying it on to be honest, other than the fact that I believe that if it's a small bomb and it only ruptures the hold, there's a greater chance of the passengers surviving provided the pilot can plop it down reasonably well.