Rules-wise, you only need to be aware of it's presence (plus abide by any flaws specific to the use of the item). Mechanically, you need to have a short OOG conversation to let the player know you're activating their magic item, and that they do, indeed, have that item on them. Maybe you think you're activating their Life item that they "always" have in their pouch but it turns out "always" doesn't include "right now", maybe they already used it, maybe they dropped it, maybe someone picked their pocket, etc.