What can be said is that he was alone, he was impoverished, and he died in a hospital without family nearby. His death, like much of his life, was invisible to the structures of record-keeping and commemoration that would later reconstruct it.
The coroner who processed his death was Major Sir William Bentley Purchase, whose office had an informal contact with intelligence-connected figures in London. The arrangement was not formal, but it existed: Purchase would alert certain intelligence contacts when a death with useful characteristics came through his office. The useful characteristics in this case were approximately the right age and physical build to pass as a military officer, and a death by causes that would not be immediately apparent to a Spanish doctor examining a body that had been in cold storage and then in the sea.
The phosphorus poisoning was relevant to this last point. Phosphorus leaves traces in the body’s tissues, but those traces would not be easily detectable in a general examination by a physician without specific toxicological testing. A body showing the marks of time in cold water might reasonably be attributed to drowning by a medical examiner who was not conducting a full forensic investigation. The plan required that Spanish authorities, finding the body of a man who had apparently drowned, would not conduct detailed tests that would reveal a different cause of death.
Sir Bernard Spilsbury, who was consulted on these questions, apparently indicated that a body kept at low temperature after death could be preserved for a period of weeks in a condition that would be externally consistent with drowning without immediately revealing a different cause. This was a matter of some delicacy. The cover story for Martin’s death was that he had drowned after an aircraft crash at sea, and his body needed to be consistent enough with that story to survive a routine examination. The fact that Michael had died of phosphorus poisoning, rather than drowning, was a risk that was accepted as manageable.