The tensioner operates hydraulically. So probably it is part of an anti drain down check valve. No illustration of the passages inside the casting. So there is no way to be positive.

Although years ago it was recommended you prime the tensioner by manipulating it after installation. I had to assume this just got some engine oil into it. This tensioner was eventually replaced with the ratcheting type. Also adjusting the tension hydraulically.

If the ball hit the ground for example a strong magnet should find it with a little luck.