Sounds like the air bled off while it was sitting. GN, NP, and BN rules all said that air brakes alone should never be relied on to hold a train parked on a grade. If the train was stopped for any length of time, enough hand brakes should be set to hold the train.
If the train was in a siding, waiting for a meet, using just the air brakes would be acceptable, but then they wouldn't be in emergency, and it shouldn't be for two hours. Once the train is in emergency there's no way to recharge the brake line without releasing the brakes, and eventually the pressure in the reservoirs will be depleted by leakage in the cylinders. At that point the brakes release on their own, and you're moving.
I don't know if any railroad routinely uses retainers today, but that's what they were for - keeping a brake application while the train line and reservoirs were recharging. Very handy on long grades before dynamic brakes.
