I often see steel wool stuck to pickups after it was used to clean and polish the fingerboard and frets. If a repairman flips the guitar over on a workbench to wire the back cavity, the steel wool hidden in his workbench carpet will be attracted to the magnets and pole pieces. Steel wool is a mess to a repairman working on pickups because it sticks to everything inside a pickup. I’ve seen steel wool between the string and pole piece. You should hear the note it makes when you play! Anybody that works on an electric guitar should have a clean area free of materials that could contaminate the electronics in an instrument.