An F-35A and F-35B during weapons delivery accuracy testing. Lockheed Martin/Chad Bellay

When the F-35 flies over friendly countries for overseas deployments, you may notice some strange tags on the body of the otherwise sleek jet.

What are those? Lockheed Martin

Every angle and surface of the F-35 has been precisely machined to baffle radar waves, so little notches like the ones on the picture above would defeat the purpose of the weapons system that has cost about $400 billion so far.

Here's the Marine Corps' F-35B flying clean:

An F-35Bs taking off from the USS America. Lockheed Martin

The notches, which are called Luneberg reflectors, serve a purpose. The reflectors increase the F-35's radar signature several hundred times over so that a plane that would normally be nearly impossible for civilian air traffic controllers to spot would give off a big, safe blip.

Perhaps the F-35B above didn't need the reflectors because it took off from the sea, away from potential spotters.

In addition to helping friendly nations spot the stealth jet, the markers on the F-35 may serve another, more military purpose.

In October 2015, days after Russia began its air campaign to bolster Syrian President Bashar Assad's forces, national security writer Dave Majumdar wrote in War Is Boring that Russia may have been using its anti-air systems to gather intelligence on the F-22, another US stealth aircraft operating in Syria.

"While it appears the Russians are following their standard doctrine with regard to the deployment/employment of their ground and air assets, it's certainly not out of the question to use their newer air-to-air assets as a form of 'operational testing' in the real-world environment," one senior US Air Force intelligence official told Majumdar at the time. "In a sense, we're doing the same thing with our F-22s."

REUTERS / Business Insider

Russia operates the same ground and air assets in Syria as it does in eastern Europe, near Estonia, where the F-35 recently appeared wearing the radar reflectors.

With the reflectors throwing off and exaggerating the radar cross section of the F-35, the US could be preventing Russia from testing its defenses against the US's newest weapons system.

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