I just switched the wiring of a single-pole light switch from back wiring to side wiring and noticed that my voltage tester (non-contact pen-style one, Klein Tools NCVT-3) detects voltage in both the other black wire going into light fixsture and the ground wire when the switch is off. The tester beeped with one or two yellow lights on, so the voltage seems lower side.
I did continuity testing with my multimeter and the reading looked fine (overload when the switch was off and 0 and beeping when the switch was on).
From my completely novice point of view, the switch should cut off any current when it is off and voltage should be detected in the hot wire only but not in others. Is it not true? Does what I see indicate something's wrong or is it actually normal (like picking up voltage from the nearby hot wire)?