Scientists cannot rule out if spy Gareth Williams was poisoned before being dumped in a bag because it took MI6 a week to realise he was missing, an inquest heard today.
Traces of a drug matching the party drug GHB appeared in his bloodstream - but forensic expert Denise Stanworth said it "probably" appeared naturally after he died.
She added that "we cannot rule out volatile agents" as she was asked how reliable toxicology can be nine days after death.
Ms Stanworth also said it was impossible to say if there was any sign of the legal high poppers, acknowledging that the drug "could have caused loss of consciousness or death".
MI6 had earlier apologised for failing to raise the alarm about his disappearance, conceding the error may have hampered police inquiries.
Relatives walked out on the inquest in tears as a senior spy said the secret service was "profoundly sorry" for delays in noticing he was missing.
The family's lawyer accused MI6 of showing "total disregard for Gareth's whereabouts and safety" before he was found dead in his London home on August 23, 2010.
Mr Williams, who was largely teetotal, had 78mg per 100ml of alcohol in his blood when toxicology tests were carried out on his decomposed body.
Ms Stanworth said the traces of GHB was "likely to be post-mortem production".
She added: "If someone had taken GHB close to the time of death you would expect the urine level to be much higher.
"I can't completely rule out ingestion of GHB but it would seem unlikely.
"In my opinion it's far more likely to be post-mortem production."
When asked how reliable tests could be, Ms Stanworth added: "In terms of many of the drugs, reliable, but in terms of the more volatile substances, not that reliable."
MI6 earlier denied "dark arts" were behind his death, despite saying a malign or hostile party could have "theoretically put some pressure on to Gareth".
Speaking from behind a screen, Mr Williams' boss offered a full apology for MI6's slow response to Mr Williams's disappearance in August 2010.
The woman - named only as SIS F - said: "We are profoundly sorry about what happened.
"It shouldn't have happened and we recognise that the delay in finding Gareth's body has made it even harder for the family to come to terms with his dreadful death and we are truly sorry for that.
"I also appreciate the delay had some impact on the police investigation."
The spy's non-attendance at work should have been raised as an issue two to four hours after it had been noted, she said.
SIS F blamed Mr Williams' line manager for the "breakdown in communication" but said disciplinary action was not taken.
Measures have been put in place since his death that would "prevent a repeat of what happened," she insisted.
SIS F was asked by Coroner Fiona Wilcox about evidence Mr Williams might have carried out unauthorised searches.
She said: "It seems to me that there was less to this than meets the eye."
When officers finally entered Mr Williams's flat in Pimlico, central London, they discovered among his possessions some £20,000 of high-end women's clothing and shoes.
Examination of his home computer showed he had also visited websites about claustrophilia - the love of enclosure - and bondage and sadomasochism, the inquest has heard.
Mr O'Toole told the witness of speculation "that revelations about Gareth's private life might have rendered him unsuitable for SIS work".
SIS F replied that it was a "sensitive area" of questioning as it related to vetting issues but indicated his lifestyle and sexual preferences would not in themselves have posed a problem.
"I can't respond directly to the questions over Gareth because that goes to what knowledge we did or didn't have of him," she said.
"There's no set template as to what (an employee's) lifestyle should be. Individuals have lifestyles and sexual choices which are perfectly legitimate.
"Our concern in the vetting process is to identify whether anything in the individual's background, lifestyle, creates a risk for him."
The inquest was adjourned until tomorrow.