A central part of taqwa is that the standard for conduct does not change depending on who is watching. What violates another person’s rights in public can violate them just as surely in private.
Example 1: Do not use someone’s property without permission even if nobody is present.
Example 2: Apply the same honesty and respectful speech online that are expected in face-to-face interactions.
حَيْثُ مَا broadens the scope to wherever a person is. The addressed form كُنْتَ keeps the message personal: taqwa is not restricted to one setting.