Does this mean that anyone who comes into possession of the QR code can load the seed on any other device or only on SeedSigner? I admit that the idea that the device itself has nothing in it that could enable a potential thief to hack it sounds very good, but is there an option to protect the seed with a passphrase in case someone does find the backup?
I don't know if SeedSigner uses encrypted QRs, but I use Krux, which is like a deluxe SeedSigner. Krux lets you create encrypted seed QRs, and that's an awesome feature because it means if somebody finds your seed QR, they won't be able to read it. Without the decryption password (or passphrase if you want) it's unreadable. That's an amazing feature. If SeedSigner doesn't have it yet, I have to assume they'll add it.
Check out Krux. It's fantastic.