[mothered]

Sealed box seems risky to a certain extent. Obviously they can find the item is not there / has been replaced but can not prove that I had any doing in it.

Obviously they can find the item is not there / has been replaced

^ Not at all.

The point of the sealed box method, Is to return the box as per Its manufacturer's state. If there's no signs of tampering nor any type of Inconsistencies, then they'd have no reason to check. As such, they'll place It back Into stock with the rest of their Inventory and a refund/replacement will be Issued thereafter. What happens from this point forward, Is not tied/linked to you.

I've performed this on countless occasions (both online and In-store) without fail.

So would you place another item inside of the package at that point and reseal the item then?

You can add just about anything you like- they're not going to check It.

Although In-store returns (that're physically handed to the customer service counter) are not weighed there and then, It's good practice to replace It with something of similar weight, just to cover all angles.