The present invention relates to a device for a portable user apparatus, including a portion which can be folded over the user apparatus, said foldable portion including at least two detection layers, each kept at a guard potential and including at least one capacitive detection electrode: a first of said layers being arranged to carry out a function of capacitive detection of at least one control object located on one side of said foldable portion; a second of said layers being arranged so as to carry out a function of capacitive detection of at least one control object located on the other side of said foldable portion, with a view to triggering a function in the user apparatus. The invention also relates to an apparatus provided with such an accessory device.