Observation pure of the Observer's presence
My point being that we make great pains to avoid disturbing our subjects when studying them, so why wouldn't ETI observers do the same? I'm not assuming they're like us, but if they're here they've come a long way to learn something new about the Universe, and for them the effort needed for concealment - compared to interstellar travel - is miniscule. If they want experimental subjects - well, how many thousands disappear each year and never reappear?