"It is not precisely that he hasn't noticed-- he certainly has been aware that his boxes have lacked cheese packets. But I was, at first, only removing them occasionally, to reduce his intake."

A box here, a box there. Slowly increasing the number that he removed, to gradually lower Dimitri's consumption of powdered cheese product. Also, technically, steadily increasing his reliance on Dedue to provide the substitute for his meals, but Dedue has long been preparing his liege's meals. It's fine, probably.

"I am certain that he will eventually become suspicious. But there is a saying in Faerghus... how does it go? 'It is easier to beg forgiveness than to seek permission', I believe."