Fast idle speed is adjustable on some buses/engines, typically the ones that use the cruise buttons for engaging/setting high idle. Hold the accel/decel button up or down in order to set idle speed at a certain rpm. I believe that feature is for adjusting PTO speeds on certain models. Other buses just have a high idle toggle switch. I have no idea what yours uses.
The part air is hissing from doesn't do anything with brakes, it's likely the pressure regulator for the stop arm or air door. If it's the stop arm, and seeing how this isn't in school bus service, I'd follow the hose to wherever it goes near the tank and plug that port, eliminating the hose/switch/regulator and any future leaking.
If your bus has an adjustable high idle speed, the mechanic did likely "change" it in order to speed up air pressure build up after they dumped the tanks. Nothing malicious there. Service writer probably had no idea, as it's such a minor thing not worth mentioning in a work order. Not a big deal IMO.
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