Asklemmy

you're looking for steganography; I'm not aware of any steganographic constructions involving qrcodes, but there could be some. generally speaking, stego becomes practical in settings where there is unimportant high-entropy data which can be replaced with an encoding of the (typically encrypted) hidden message without altering the "cover" message in a detectable way. it might seem like this is the case with qrcodes due to the error correction; if you change a few random parts of the qrcode, due to the error correction it might still decode properly. However, this is probably easily detectable if the adversary has a high-resolution image of a candidate "maybe stego" qrcode: if decoding and re-encoding it produces a different qrcode, that would indicate that there had been extra data hidden in the error correction. (or, using a more advanced decoder than the typical ones, they could also simply see what the error rate is.)