I think you're confusing all metal (threshold based) with a hum and disc mode at minimum. If it's an ED180ish detector like the compadre, Mojave, Cibola (more like ed165) vaquero and some other older models, turning disc knob to minimum will allow almost all metal to beep, but its still in "silent search" disc mode.
All metal (threshold based) is more like a live feed. The threshold is set to a slight hum and you will constantly hear it and any time a metal target comes under the coil you will get a response.
To further the description, threshold based all metal comes in different flavors too. The cibolas and vaqueros pinpoint button momentarily puts it into a "fast auto retune" threshold based all metal. So if you go over a target and hold the coil there, it will respond but quickly retune the threshold by itself to stable hum.
The vaquero has a "drop down" all metal I call it. By turning the disc knob completely to minimum, you will "click down" (similar to how you click on the detector) into a slow return all metal. This means it will act the same as the cibola or vaquero in pinpoint but will retune itself more slowly to the constant stable threshold hum.
Then comes the full all metal and not auto reutune all metal found on some older detectors like the bandido II micromax and the still produced outlaw.
On these and other detectors there is a pinpoint fast retune all metal by pushing button, a switch for "auto" all metal which is similar to the "drop down" all metal of the vaquero that is more of a slow auto return all metal. But the third variant is just called "all metal" on the detector. This is a true all metal in the sense of retune. It will not retune itself. If you go over a target and stay it will continue to scream at you until either you move your coil or you use the "retune switch" (bandido II micromax) or the "retune button" (outlaw) to manually return the threshold back to the slight hum you previously set it to. This mode I like best to see the size of the target by tracing out it's edges. On some targets it can be done in auto retune all metal but you can't always be sure you found the edge or the detector returned itself.
Hopefully this isn't confusing. But I wanted to clarify what the differences of all metal is.
On the bandido and outlaw they are ED120 machines, so with disc knob at minimum most iron is already blocked out.
So depending on which machine you are running, minimum disc can allow almost all metal to sound off, but on others minimum will still block out iron.
But the cibola, compadre, Mojave, eldorado, tejon and some others having disc at minimum will allow any metal to have a response, it's still in disc mode.
