A woman's wig hanging on a chair caught the attention of the policeman sent to Gareth Williams's flat to check on the missing spy, an inquest has heard.
It was unusual that he was not overwhelmed by the smell of rotting corpse, said Pc John Gallagher, as he was the first officer to set foot in the MI6 spy's home.
He was on foot patrol at about 5pm on 23 August 2010 when he received instructions to carry out a "welfare check" on Mr Williams, who had not been seen for about 12 days.
He went to Flat 4 in 36 Alderney Street in Pimlico, central London, half a mile from MI6's headquarters, and was let into the building's communal hallway by a neighbour.
MORE FROM THE GARETH WILLIAMS INQUEST
The officer went upstairs to Mr Williams's top-floor apartment and knocked repeatedly on the door but there was no reply.
Pc Gallagher spoke to his control room, who got lettings agent Vanessa Scott to come to let him into the flat.
The officer went through the rooms checking for any sign of Mr Williams.
In the living room he found a mobile phone and two SIM cards on a table, then spotted something else that caught his eye.
"My attention was drawn to a lady's wig hanging on one of the corners of a chair," he said.
There were some yellow sports holdalls in the bedroom, and the duvet cover was half off the bed, trailing on the floor.
There was also a neatly folded pile of clothing on the bed and a bathrobe on the floor.
He eventually walked into the flat's bathroom, noticing a bulging red North Face holdall in the bath with the zips padlocked together.
Pc Gallagher said he only picked up the "particular smell" of a body when he tried to lift the bag up.
"It is unusual because normally you would expect to smell it earlier," he said.
"I looked at the bag to try to get a picture. I noticed that the side nearest the door had a round bulge.
"I noticed there was a padlock with the two zips joined together.
"At this point I am realising it is something serious and my concern was to not damage anything in a crime scene."
He lifted the bag up "by six or seven inches" when he could see red fluid seeping out.
"Then there was the smell," he said. "Probably as a result of moving the bag."
Pc Gallagher said he called for assistance from officers, saying he did not have the appropriate equipment to look inside the holdall.
Detective Sergeant Paul Colgan was sent to the flat as back-up. After cutting into the bag, he confirmed there was a body inside.
He recalled: "At that point I knew we had some sort of a crime scene, so I ordered everybody out of the flat.
"Once we were out in the street, I called for other units to attend."
Officers from Scotland Yard's counter-terrorism command, known as SO15, were sent to the flat that night, the inquest heard.