Could it be that the stamps have an invisible UV coating on the surface and as the stamp(s) passes thru the cancellation process or machine they are briefly illuminated by a UV lamp and a glow is detected by a censor, and once detected, the stamp(s) will be cancelled...?

If the process of cancellation was missed, do you think a letter carrier has a device to hand cancell the stamps...?

Do you think such things are possible...?

JB