The woman, who was named locally as Isabel Rivera was last seen alive in September 2004, according to neighbours in the apartment block in Ciudad Lineal district of Madrid.
On Tuesday, police finally gained access to the woman’s home with the help of fireman who entered her apartment via a second floor window.
Neighbours said that years ago they had reported “a strong smell” emanating from the property but their complaints to police hadn’t been followed up.
The woman was reportedly a widow who had no children and lived alone and was estranged from her sisters.
But it was a niece who finally contacted police to ask for help locating her and that led to the discovery of her corpse.
Reports suggest that rather than decompose, the conditions of the bathroom where she was found had caused her body to be conserved.
Initial investigations suggest that she had died from natural causes.