You can create bad blocks then store data there (NAND memory). Bad blocks are ignored, so you are effectively invisible.
And in use for more than a decade too! Its not just applicable to NAND memory. You can do this to CDs and DVDs as well.
In the old days when they were first getting pissy about copyrights and sharing games and software, I found that they had been using a rather clever antipiracy mechanism.
What they would do is create the CD to intentionally contain a couple of bad blocks.
In normal usage the drive would never attempt to access these blocks, as the software would elegantly skip around them. But when you tried to copy the CD it would get about 70% complete and then hang, taking so long to try and salvage data from the bad blocks that it would buffer underrun the burner and ruin the copy being made.
RFID is another scary can of worms in and of itself. If you even get close to being able to manipulate it without all kinds of licensing red tape, they are really quick to lawsuit you to death. Its inherently flawed in a very serious way, one that enables anyone with the right kind of equipment to read it at will. And its only a matter of time until viable designs for that equipment become well known to the public, rendering RFID a completely worthless concept.