Agreed, not in NEC.
A propane regulator will be a worse case than just tank (except when filling) or a meter because the regulator has a vent for release of excess pressure on the low pressure side.
However, propane is heavier than air unlike natural gas, and will collect in low spots rather than rising.
Try to find a specific rule for propane regulators.
There is no such specific rule for regulators in NFPA 54. If you want to find one somewhere else - go for it.
That propane is heavier than air is irrelevant to this issue since there is no particular "direction" of the release. As for that matter, methane dilutes and disperses rapidly. Three feet is a very small hazard radius for either gas.
Generally what's considered a source of ignition?
In the NEC, Article 500 world, a circuit breaker doing its job as a fault clearing device would be considered a source of ignition (it doesn't have to be constant). As mentioned above, the hazard radius stated in NFPA 54 is actually quite small for any fuel gas, but it is considered adequate by the technical committee in control of the Standard.