Ah right.Reading the mind website the definitions and explanations are quite scary. Stuff like free to go home, 12 month detention renewals etc terrifying in itself without mental illness on top to deal withIn reality there is a very big difference under voluntary you can refuse treatment under section 2 you can't, but in terms of detention you're right even if you're voluntary your not leaving without assessment and there are further short duration section powers for medics and nurses to prevent you walking out until that happens.
Section 136 is a specific police power to take to a place of safety if in a public place without application for a warrant (s135 is removal from a dwelling and needs warrant). You would then be assessed and come under a section 2 or 3.
You dont have to be arrested under 136 to be assessed (say if you're in outpatients already or professionals are called to your home and given entry freely by you or another) its mainly just right to transport against your will for assessment