This is a great question because it hits at one of Orwell's hidden gems about his writing. Orwell's use of syntax at this point in the novel is significant for a couple of reasons. The first is its location. The sentence is at the end of chapter 2. It is at this point at which the animals have declared and obtained their freedom from Jones and the other humans. There is the hue of revolutionary glory. In this, though, emerges the issue of leadership and what this will entail in a post- Jones setting. Whereas the hierarchy in the Jones' days consisted of animals being subjugated by humans, Old Major's vision is one in which all animals are to treat one another fairly and equally. The issue of the milk is one of the first issues where this edict of Old Major will be repudiated, as the hierarchy begins to emerge with the pigs being at the top of it. Orwell does not want to bring this out immediately, but rather build to it.
In this vein, the syntax of his description is significant. In order to evoke the ember of disillusionment and betrayal of revolutionary ideals in the consolidation of power that results, small hints are given to the reader about how there will be a difference in the aspiration to have power and the actual execution of it. Orwell's choice of syntax and phrasing help to reveal this. The idea of that it was "noticed" helps to convey the idea that someone, perhaps insignificant, noticed the missing milk. More pointed would be that the milk "had disappeared." Orwell's syntax helps to construct the nebulous propaganda descriptions of the Soviet Union at the height of its power. The manner in which governmental "company" explanations failed to effectively explain anything, but indicated that something had happened. Orwell borrows this same syntax approach in order to strengthen the link between the leadership on the farm and Communist Russia. It is through this syntax where a connection is forged and strength in the narrative is evident in Orwell's prowess as a writer.