Isn't this just a veil? Put up a block against alertness and that stuff won't get noticed easily. Harry isn't great at this sort of thing, but he CAN do it.
Use "Getting Selective With Conditions" PG 277.
Arguable an item that does a veil COULD last until next sunrise, I suppose (perhaps add a caveat that this is only true as long as nothing overt is done with the item). Or probably better (e.g. more balanced) to go with the base duration for Evocation (a few moments) and use the Thaumaturgy time increments to increase the duration. Hmm, that might not get the job done with Harry's lore though. Let's say he can make a Strength 6 item due to specializations and such. Subtract 2 to make him immune to the veil's effects, that's still strength 4. That's enough to get by a standard guard. IF the GM rules such things last until sunrise like a thaumaturgical veil, then that's pretty good (quite possibly too good though).
Hmm, I suppose if I was a GM, I'd lean towards making the default duration for an enchanted item doing a veil be something like "a few moments" and then each bit of strength spent increasing the duration would move it up the time chart. Then I'd give a bonus to strength (spendable on duration or potency) if the item was smaller than a person and only affected itself. Hmm, maybe I'd use the lifting chart as a baseline, so perhaps you'd get 4 extra power to the effect for something like a knife (I figure that's well below a loaded backpack). That immediately gets you a knife that can hide at your lore for 15 minutes when activated. A small satchel (say half the size of a backpack) could hide itself and its contents for a few minutes. That feels about right.
