For several weeks Margi has corrected mainly chalk signs on sandwich boards.
She says signs with errors are endemic throughout the city.
Down at the River Traders Market one Saturday she found 10 signs with either spelling or punctuation errors.
Margi doesn't do what she does out of spite, rather from a love of the English language.
"Our beautiful language is being dumbed down," she complains. "Signs are important, they convey important messages. Errors can sometimes change the meaning entirely."
"Corrections annoy people. They feel hurt, or dumb, and that's not my intention.
"Not everyone is a natural speller and I'm not trying to put them down for that.
"But when you publish, or put up a public sign, there comes a responsibility to be correct."
Grammar cop Margi Keys
Not surprisingly Margi has a background in teaching - and enjoyed marking exams.
But it was another grammar vigilante in Bristol, United Kingdom, who was her inspiration to strike out with a sharpie of her own.
Wielding an 'apostrophiser' - a broomstick with sponges and stickers - the unidentified grammar vigilante corrects badly punctuated shop signs in the dead of night.
Margi's friends now send her, via Facebook, examples of poor grammar.
People don't know what they don't know, she says. By drawing attention to correct grammar she hopes she may somehow have a positive influence.
Margi has not been confronted by sign owners, yet, but has received criticism on her Facebook page, but says her cause is just. "Court cases," she says "have been lost on incorrect punctuation."
Margi says she is not perfect and makes mistakes all the time. Smartphones and auto correct have a lot to answer for she says.
In one text message that she sent to a friend the word bigger became a similar word only with a u instead of an i, and another friend sent her a text complaining of a crock in her neck.