Item patches are paper platforms with question marks on them found in Mario & Luigi: Paper Jam. Mario, Luigi, and Paper Mario can uncover various items hidden in them by using Trio Drill on said platforms and then popping out. There are 16 item patches in total. Peach's Castle, Bowser's Castle and Neo Bowser Castle are only locations without any item patches.
Item patch locations
Doop Doop Dunes