More than welcome Richard.
In above case and IMHO, HWH should be placed "Out of Service" until it is determined to be set up for use with propane.
When using standard Caution, Warning, and Danger (as seen often in manufacturer install books), IMHO, an unknown gas type / product set-up would fall in the Danger Category.
Revise my original - "Needs to be looked at by service professional" to "Discontinue use until looked at by service professional".
Full Disclosure: I am not a Home Inspector, so I do not understand what SOP would be in this case, Just offering a humble opinion based upon my limited knowledge of the difference between Nat Gas and Propane properties.