They would figure it out. Under the faceplate there is a warranty sticker, and removing the sticker by force or opening the case with it on will indicate that you opened the console.
To remove it successfully you have to heat it up with a hair dryer and then use a sharp knife to carefully pull the sticker off while the glue under it is still hot. You then have to put the sticker on wax paper and repeat the process when you want to put the sticker back on the 360. However, keep in mind this constitutes warranty fraud, which is illegal.