Update 11.15am, July 25. Muriel has now been found.
Police believe that a missing 85-year-old woman from Boston is still in the area.
Muriel Addy, also known as Biddy, was last seen on Norfolk Street at 5pm on Monday, July 24.
She is 5ft tall, of a slim build with grey hair and glasses.
Anyone who knows where Muriel is should call Lincolnshire Police on 101, quoting incident number 84 of July 25.