After reading all the replies until yours, I was surprised to have to wait so long for the "correct" answer.
To be clear, I'm talking about a traditional Fender Precision Bass. P basses with onboard preamps, newer pickup designs... these may behave otherwise.
For many years I have had the habit of rolling back the volume knob of a Precision Bass just a tiny bit; maybe an 1/8" or 3/16" back. The sound of a proper Precision pickup at full volume is distinctive in its increased midrange and treble output. Rolling back the bass's volume control attenuates that part of the spectrum enough to matter.
Once, hanging out in the old Nordstrand shop, I was playing my vintage Precision after Carey and Maurilio restored it. Carey noticed my setting it's volume this way and asked about it. When I said why I did it, he said that he had another middle aged customer who did the same (I was pushing 50 at the time).
The player's age may inform the likelihood of their intentionally using this technique. I believe that in the current era of "cutting through the mix", bass pedalboards, and multiple preamps, some such nuances of traditional equipment are vanishing from knowledge.