Below is the Wikipedia entry for Alger Hiss that refers to General Kobyakov's search of the Soviet archives for any proof that Hiss was a Soviet spy. The highlights are mine.
I'm assuming that the Wikipedia entry covering the search of the archives describes the proof that you are referring to that supposedly "proves" that Alger Hiss was not a Soviet spy. In fact there is a mountain of evidence proving that he was and I'll be happy to provide some of it since it has mostly escaped the MSM reporting on the Hiss case.
The key thing you should get out of the Wikipedia entry is that a total of two days were spent looking through a massive Soviet archive looking for Hiss material. This in not a two day job. The Soviet archives naturally refer to Hiss by code name and you have to do the necessary detective work to identify who is who in the various reports. Without this additional effort any search would be worthless and could only turn up uncoded references to Hiss. The Soviets were much too disciplined to make that mistake. There were other non-Soviet services that did make that mistake, however, and that is a part of that mountain of evidence that I refer to. These eastern bloc archives refer to Hiss by is real name and they are consistent with Soviet data.
In fact most of the supporters of the "Hiss is Innocent" theory have given up on defending Hiss and have admitted that the evidence is just too great to any longer be denied.
From Wikipedia:
"Soviet archives
After the dissolution of the Soviet Union in 1991, Alger Hiss petitioned General Dimitry Antonovich Volkogonov, who had become President Yeltsin's military advisor and the overseer of all the Soviet intelligence archives, to request the release of any Soviet files on the Hiss case. Interestingly, both former President Nixon and the director of his presidential library, John H. Taylor, wrote a similar letter, though the actual contents of those letters are not publicly available.
Russian archivists and researchers responded by reviewing their files, and in the fall of 1992 reported back that they had found no evidence that Alger Hiss had ever engaged in espionage for the Soviet Union or any evidence that Hiss was a member of the Communist Party. However, Volkogonov subsequently revealed that he had spent only two days on his search and had mainly relied on the word of KGB archivists. He stated, "What I saw gave me no basis to claim a full clarification. [Hiss attorney] John Lowenthal pushed me to say things of which I was not fully convinced."[22]
General-Lieutenant Vitaly Pavlov, who ran Soviet intelligence work in North America in the late 1930s and early 1940s, provided some corroboration of Volkogonov in his memoirs, stating that Hiss never worked for the USSR as one of his agents.[23]
In 2004, General Julius Kobyakov, a retired Russian intelligence official, revealed that he had been the person who actually searched the files for Volkogonov. According to Kobyakov, his research revealed that there was no indication that Alger Hiss had been either a paid or unpaid agent of the Soviet Union only "after careful study" of KGB archives and "after querying sister services" (military intelligence)"