SRS systems don't depend on speed of impact to deploy, they use accelerometers in the car to detect acceleration rates. (FYI acceleration is the term used to describe a change in speed, both faster and slower).
It is possibble for the airbag to deploy when you are parked and ignition on, when someone drives into you from the front.
Most DTCs have a self-reset function built in. For example, if no fault is detected after 255 key starts, DTC is deleted from active/current code memory, but stays in history code memory, and warning light switches off. This is why your airbag light switched off after a while.
All SRS functions are disabled when the warning light is illuminated. This is a safety feature to minimise risk of false deployment when a fault is detected with the system.