Before we had the instruments to observe them directly we could theorize about the existence of bacteria because we could indirectly observe them through their effects on our biology and even their macroscopic effects on populations, effects that had no better explanations. I am not aware of any mysteries that are
most simply explained by a hitherto unobserved, technologically advanced (I assume we're not talking about dolphins when we say) "non-human intelligence", whether they supposedly dwell in the depths of the ocean, the Earth's crust, Titan, or anywhere else in the universe. SETI has been listening for ~60 years and hasn't heard a peep from any of the billions (trillions?) exoplanet's worth of radio signals that could have reached us in that time.
The available-to-me evidence suggests that technologically advanced species are exceedingly rare, and the only such species we're aware of emits an overwhelming number of artifacts that would serve as evidence for its existence, so it would be very much not mundane to discover that another one has been living under our noses this whole time.
I am not making a truth claim here, as in "it's definitively untrue that there are non-human intelligences sharing the planet with us," I'm just arguing that it's an extraordinary claim that should require extraordinary evidence - grainy footage and hearsay isn't enough for me.
The available-to-me evidence suggests that technologically advanced species are exceedingly rare, and the only such species we're aware of emits an overwhelming number of artifacts that would serve as evidence for its existence, so it would be very much not mundane to discover that another one has been living under our noses this whole time.
I am not making a truth claim here, as in "it's definitively untrue that there are non-human intelligences sharing the planet with us," I'm just arguing that it's an extraordinary claim that should require extraordinary evidence - grainy footage and hearsay isn't enough for me.