> "We have no other way of detecting except through the same effect we use to infer its existence" is true of everything in the universe

No. For example, staying within astronomy, we detect gravitational waves and if we find an optical event that correlates with the gravitational waves, it is multimodal (we are at n=1), and if you don't solicit events (or if you statistically blind it by adding fake events in your solicitation) it's multimodal independent (currently n=0). Going outside of astro, you'll find very rich multimodal studies "I know what this molecule is because I have an IR, UV, NMR, Mass spec, etc etc etc"