In early April the Syrian Assad government apparently used chemical bombs on a rebel controlled village. The attack killed 89 people, most of them civilians, including 20 children. The victims showed symptoms of nerve gas being used and subsequent chemical analysis confirmed this. The Syrian government accused the rebels of making the attack or having stored chemical weapons in one the buildings the bombs hit. But the U.S., NATO and Israel soon confirmed that it was the Assad forces who delivered the nerve gas. Russia continued to back the Syrian government but several major intelligence agencies worldwide agreed that the evidence against the Assads was pretty compelling. Even the Chinese agreed and refused to go along with a Russian effort to block a UN condemning the Assads.
There was more to this than was reported. For several years Israeli intelligence analysts have quietly urged their Western counterparts to take a close look at who still had chemical weapons in Syria because there was general agreement that they were still being used. Everyone blamed the Islamic terrorists but the Israelis pointed out that a growing number of those incidents appear to have been the work of the Assad forces and the Israelis were getting information indicating that the 2013 Russian brokered (and guaranteed) deal to get the Assads to surrender all their chemical weapons was not being observed or enforced.
It is still unclear how much the Russians know but Western intel agencies were paying more attention to who was saying what to whom about chemical weapons in Syria before the April 4th nerve gas attack in Syria. After April 4th priority was placed on analyzing a lot of the collected, but not thoroughly analyzed, data (mainly communications intercepts) the U.S. had. There has long been a problem with more data being collected than could be thoroughly analyzed but nothing is thrown away. Now that there was a definite use of nerve gas (medical experts from several nations agreed on that) the deep dive into the intel data was ordered and it did indeed find evidence of the Assads retaining stocks of chemical weapons, including nerve gas, and apparently using it in the belief that they could get away with blaming the rebels. That gambit fell apart under close examination and now Russia (and to a lesser extent Iran) find themselves in an embarrassing situation. Russia supplied support for the Assads to find targets and the Assads used that to find the target for the nerve gas attack. Iran is one of the few nations to use nerve gas in combat (during the 1980s war with Iraq) and has experts in that subject who may have been advising the Assads. After all, Iran has openly discussed using nerve gas against Israel, which is why Israeli intel continued to monitor the nerve gas situation after 2013.
The most famous example of catching up with deciphering data backlogs occurred after World War II when American code breakers decided to decrypt messages sent by the Japanese government between the time Japan surrendered and the U.S. occupation forces arrived and took control of all government offices. Some of these decrypted messages were shocking. That was because Japan had secretly decided, according to the decrypted diplomatic messages, to play the victim. This was a long-term plan and the Japanese government had come up with it even before their surrender in 1945. In part this was to create some kind of defense for war crimes prosecutions that were already underway against German officials when Japan surrendered. Thus Japan sought to play the victim when it came to their responsibility for horrific wartime actions. The secret messages to Japanese embassies after the surrender contained details on how to spin the fact that Japan, rather than invincible as it had long claimed, was now defeated because it tried to keep the evil West out of Asia. Japanese diplomats were ordered to play up the “Japan is a victim” angle. While this played well in Japan, it annoyed and disturbed the neighbors. Some Japanese understand what is going on here, but because the Japanese educational system is based on this 1945 decision (which played down Japanese atrocities in Japanese textbooks and media in general) most Japanese cannot understand the reaction of foreigners when it comes to Japan and World War II. Searching through raw intelligence backlogs has become more of an issue since World War II because a lot more information is being collected and only a fraction of it can be decrypted or simply translated and searched. This is one of the dirty little secrets of the intel community and it continues to resurface when least expected.