I asked my doctor about this once, because my leg didn't move very much when he'd do the test. He said "I can see a very small movement, which I confirm with a second gentle thump. I could hit you harder, but it wouldn't be very nice."

And then he hit me harder and it wasn't very nice.

The lower leg moving would be an obvious sign, but they may be watching above the knee (where the muscle is actually contracting to create that movement) for even a twitch. Even absent that, without other symptom complaints they may just not be holding much weight with that one test. If you came in complaining you recently started dropping things, tripping, or injured yourself in a fall, that would carry more weight.

You can try it yourself, sit in a chair or the edge of your bed, and tap your knee just below the kneecap with your fingertips. You should be able to feel your leg twitch.