Detectives discovered the body of an 80-year-old woman at a lock-up at the site in Colindale around 2am on Tuesday morning.
Officers combed the area in Sheaveshill Avenue during a search for a woman reported missing just 20 minutes earlier.
She was confirmed dead at the scene.
A post-mortem examination took place at Northwick Park Hospital on March 1 however, Scotland Yard were unable reveal a cause of death.
Detective Chief Inspector Noel McHugh said: "At this stage we are keeping an open mind about any possible motive and would ask any witnesses or anyone with information to come forward.
"I make a direct appeal to anyone with links to these allotments who would have visited or had been in or around the area throughout Monday, into the early hours of the following morning.
"An elderly woman lost her life in the most tragic way possible and was left inside a storage unit similar to a garage. If you know anything please contact us."
Anyone who can help is asked to contact police at the incident room on 020 8785 8099, via 101 or on Twitter @MetCC.
Alternatively, contact Crimestoppers anonymously on 0800 555 111.
