I completely disassembled mine and there was no warranty sticker whatsoever.I have to mention though that it had it's fans and display changed under warranty policy so it has been previously opened by an authorized ASUS repair center so I don't know if any stickers were present beforehand.So in my case I don't think they can prove in any way that I have opened it up.I don't think they put stickers because it is quite hard to disassemble if you are not good with your hands and have a proper set of tools so they just naturally assume nobody would bother.