Questions remain unanswered following the death of a 'beloved' father.
Adam Holmes - who was described by his family as a 'victim of drugs' - died after taking a quantity of methadone.
Mr Holmes, 35, was found dead at his home on Castle Drive, Somercotes, on July 30 last year, an inquest at Chesterfield coroners' court heard.
A police officer told today's inquest there was no evidence to suggest any third party involvement in Mr Holmes's death - but his family raised concerns during the hearing.
Mr Holmes's brother Dale Holmes said: "We can't find the key to his door, the police can't find the key.
"We think his door was locked externally and the key was taken away by whoever locked it.
"It seems very suspect that the key is missing.
"There's been no investigation into where the key could be."
Detective Staff Investigator Kathyrn Sharratt, of Derbyshire Constabulary, admitted: "We don't know where the key is.
"That is of concern."
She added: "There is no evidence to suggest any third party involvement or suspicious circumstances in this case."
Coroner Peter Nieto concluded Mr Holmes's death was drug-related.
He acknowledged there are 'unanswered questions' and offered his condolences to Mr Holmes's family.
After the inquest, Mr Holmes's family said: "He was a beloved son, brother and father-of-two.
"He was loved by most people he knew.
"We believe he was a victim of drugs.
"Due to drugs he fell into a lifestyle most people would probably wish they wouldn't find themselves in.
"He was trying to get himself clean and on the straight and narrow."
His family said Mr Holmes liked drawing and origami.
If you need help with a drug or alcohol problem, visit www.derbyshirerecoverypartnership.co.uk