There seems to be a recurring issue with what "observable" is.
It appears to be based on the idea that if you see something with your own eyes there is no better evidence for something. And that "observation" can only by by eye balls at the scene.
I think both of these ideas are wrong.
1) Personal naked eye witnessing.
The testimony of eye witnesses has been shown to be very unreliable as a source of information.
If you witness an event you are only able to see it from one angle, with what light is available. If you could have a video record of the same event from 6 different angles would that not be better than an eye witness view. It would see things not otherwise visible and would be available for slow, step by step examination by anyone.
2) Eye Balls at the Scene
However, there maybe things which occur for which an eyewitness account would be useless. Simply "seeing" the 1980 explosion at Mt St Helen's would be pretty much useless. It would not be of any value today for example. However, the fact that the mountain was "observed" through instrumentation for months previous to 1980 is what enabled the prediction of that explosion and the ability to warn about an impending eruption now. The instruments are not "seeing" in the usual sense of the word but they do "observe".
It is not necessary to "observe" all details to be able to draw conclusions. These conclusions will be more or less safe depending on what the "observations" are.
If there is a trail of foot prints leaving a crime scene we are, in a very real sense, observing the path of a possible perpetrator. It is just not as detailed an observation as others might be.
If we detect the presence of a specific chemical in the aftermath of an explosion we are making an observation that tells us what exploded. It, in this case, may be a better "observation" that standing (at a reasonable distance ) and seeing the explosion.
It is simple not true that we can't "observe" things that happened when no one was there.
Just like the chemical explosion we can observer the big bang, for example. It left a number of different "residues" just like some explosives may.
There are a lot of ways to observe something. Some are, for some events, better than being an eye witness. It is the ability to re examine (repeat) the things which we use to observe and event which is part of science. We can never re examine an eye witness account and they have been shown to be unreliable. That is why the popular idea of "observable" is more or less useless and very misleading.
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