How does a scouter detect power levels?
I could be way off, but I sort of theorized that scouters are designed to scan for ki aura's (whether visible or not, even though all ki is actually supposed to be invisible, but manga and anime artists had to fudge this part) and then give back a number depending on how strong the aura is at the time. So if a person is able to hold back how much energy there aura is using (such as what Trunks did when he first met Freeza) then the scouter would display a deceptively low number. How the scouters actually detect ki is another mystery though, but is probably somehow similar to the way instruments in our world detect different kinds of heat or energy.
