...in his incomplete book "Extraterrestrial Presence" before he died in 1999. But he questioned the Fermi Paradox conclusion of ETI rarity because if ETIs arose a billion years ago then we could have received von Neumann probes from an immense volume - he guessed ~100 million ly. So ETIs might be rare, but there could be many here, by their proxies, anyway. If they're billennia ahead of us, then they could be observing us without us knowing, even living amongst us in human or animal form. Not that he believed in abductionism - he didn't - but he assumed the ETIs could remain hidden if they chose to, and could tailor biological machines indistinguishable from natural ones.

Why would they remain hidden?

Why do BBC naturalists like David Attenborough and his crew remain hidden and use elaborate devices to remain so? To observe their subjects as close to "wild" as possible. When your subjects are conscious, communicative beings would they ever get used to your presence? Probably not. Or perhaps we've all been conditioned to ignore the ETI monitoring devices? Perhaps they have ubiquitous code in the biosphere that "edits" out any accidental perception?

Weird, I know, but is it weird enough to be true?