In addition to the other fine points: A fundamental job of a life form is to pump entropy out faster than the environment can pump it in. This tends to eliminate any hot environments, such as stars, the insides of active planets, and one of my personal favorite science fiction life forms, neutron stars and other such exotica.

A life form must also be able to develop to some reasonable level of complexity in some reasonable period of time in order to be alive. This tends to eliminate old stand-by favorites such as nebula, very cold planets like Pluto, and the interstellar medium. It doesn't matter if they got a 12 billion year head start on us if they "live" a billion times slower than we do. (And if you start doing the math on how cold things are and how often particles interact in even a nebula, let alone an interstellar medium, it doesn't take long to see that a billion times slower is actually me being quite generous.) It is likely that at the time scales an interstellar life form would live at, the interstellar medium is too frequently disrupted by novae and such for anything to live in it.

So you pretty much end up limited down to roughly the temperature range we're familiar with pretty quickly just based on thermodynamic considerations. Given that you also have to add in the fact that chemistry must be able to support the life form, you end up with the result that life really isn't going to be that hard to identify, even if it's based on ammonia or something. It'll still be pretty obvious that something life-based is taking place.

One of the other things about this conversation is I think "a bit too much science fiction" can lead people to think that Earth life is somehow pedestrian or something, but in reality, Earth life is incredibly diverse and has covered almost every imaginable niche on this rather diverse little planet, and that makes the odds of meeting something that has literally no Earth analog pretty long. We could meet a planet of ammonia-based silicon life forms and it could very well be the case that we'd still recognize every player in the ecosystem just fine on sight, because we've got a lot of terrestrial data points as to what might constitute a viable life form. "Life forms that could exist in this universe that looks nothing like Earth life" may very well be a nearly empty set. (Or at least life that could naturally evolve. One could hypothesize a machine civilization in a lot of places, but it's hard to see how such a thing could evolve directly and biologically.)