It’s still common (even in this day and age) to have custodial personal take it upon themselves to do what they think is right. We often see this phenomena when a company goes to do a strip and finish for a new client or even an existing one. Mopping with bleach will leave the “salt” behind (typically down in the joints of the tile) waiting for it to be re-activated by a liquid.
Rinsing with water alone most often will not neutralize the bleach. It will take an additive to your rinse water like Sodium Metabisulfite. Water treatment plants employ the substance to remove trace of excessive chlorine. Sodium Metabisulfite is an effective bleach neutralizer.