The meaning has been established in another answer. I believe the meaning is more about perception than sight indeed, as in finding something or coming to a conclusion about something and this would match a specific construction for the verb voir (2e, I. C.): Voir quelquechose à quelquechose where you have a second object (indirect) introduced with the prepostion à and it feels to me a bit like "from". They're finding it's the beginning of the end from what i.e. the first something is the conclusion from the preceding sentence in the text and the second one is literally not known. I think in this very instance the second object is made up of the preposition à with an interrogation word (quoi) and is anteposed. It is only loosely connected with "je ne sais pas" whereas the whole "à quoi ils voient ça" is the object for that.