The only viable RFID would be a passive chip, which would require the scanner to be in close proximity to the notes anyway, so putting them in a mattress would

The only viable RFID would be a passive chip, which would require the scanner to be in close proximity to the notes anyway, so putting them in a mattress would be equally efficient. You could probably build a superpowered scanner that would detect the location of these notes, but I would suspect there would be so much noise in a home that it wouldn't be too easy. From a practical standpoint, should people start "hiding" their notes in their microwaves, that would be the first place to look if you're a bad guy.