Red plug is on safety valve end, so have it missing might indicate that safety valve did open at some time and that might require replacement.
Not sure what is the extend on propane tanks, but scuba tanks have visual inspections couple of times and then following inspection involves valve removal, internal inspection with camera and at the end "wet test" with water compresses to double operating pressure.
Did not scuba for several years, so don't remember the details, but exterior visual inspection can go only so far.
Than some propane TANKS operate over 50 years with no inspections at all, so maybe for ca 100 psi propane holds they are not needed?