Russian warplanes flying into Syria since early September have been watched by NATO with great interest. This proved useful because soon NATO intelligence specialists discovered that the Russian warplanes were using stealth technique they had used before. This involved having warplanes with their transponders turned off fly close to a larger transport that kept its transponder on. By flying close to the transport the usual air traffic control radar would not show enough detail to reveal several aircraft but because the radar could see that something was there and there was a transponder signal coming from that blip on the screen all that was recorded was one transport. But when NATO fighters flew up for a closer look it was obvious what was going on.
This form of “masking” was used regularly at sea during the Cold War. Russian subs, which made more noise than their Western counterparts and were even easier to detect by Western subs with their superior passive (listen only) sonar found that by moving under a larger surface ship (military or transport) they could usually escape detection by Western subs or SOSUS (permanent underwater sonar systems the United States deployed). SOSUS had a network of passive sonars on the sea bottom in key areas of the Atlantic and Pacific during the Cold War and made life very difficult for Russian subs trying to reach the high seas.
SOSUS (SOund Surveillance System) consisted of several different networks. On the continental shelf areas bordering the North Atlantic was the CAESAR network. In the North Pacific there was COLOSSUS plus a few sensors in the Indian Ocean and a few other places that no one would talk about. The underwater passive sonars listened to everything and sent their data via cable to land stations. From there it was sent back to a central processing facility, often via satellite link. SOSUS was accurate enough to locate a submarine within a circle no wider than 100 kilometers. That's a large area, but depending on the quality of the contact, the circle might be reduced up to ten kilometers. The major drawback of the system was that it did not cover deep water areas more than 500 kilometers from the edge of the continental shelf. By masking their sound with an overhead ship Russian subs could often travel long distances undetected and then mysteriously show up. Once the United States figured out this technique methods were developed to make it less effective. But the tactic never completely lost its usefulness. Now the Russians are regularly using that masking technique in the air as well.
SOSUS systems are very expensive to maintain. SOSUS managed to survive the end of the Cold War by making its sensors available for civilian research and by using cheaper and more powerful electronic and communications technology. While many parts of the SOSUS have been shut down, additional portable SOSUS gear has been put in service, to be deployed as needed by the United States or some of its allies (like South Korea).