Was it in one lobby, or do you notice it on multiple occasions?

"Crouching."

Yes, I experienced this a few days ago. Literally the same guy over and over running & gunning with UR but completely off my radar. I thought I was just trippin so I had a friend watch the rest of my match, sure enough this guy was completely off the radar while running the entire game. I watched him run right past a doorway I had just crouched behind (most likely looking for me) and not even a hint of red while he was less than 10 feet (give or take) away. And yes, I'm aware of crouching. Crouch radar invis was nerfed. Now if you crouch, you can see your center arrow disappear and reappear and same on your radar vs a crouching enemy, flash of red, then gone and then flash of red and so on.

When was the last time you had it serviced? The top radar manufacturers suggest getting a tune up every 3,000 deaths.

That's called crouching or invisibility