Hey Tom
I guess you do not bird much near the lakeshore in durham region. Northern mockingbirds are easily found when one looks for them.
Throughtout the GTA they love the habitat along rail lines (i.e. shrubs, brambles, scrub) lots of food and cover. With that said current territories I know of are Newcastle hwy 115 and cp rail line, darlington provincial park, oshawa second marsh (2-3 pairs),and the stronghold "stevenson road oshawa west to whitby harbour/brock st. north to the cp rail line and south to the lake" this rectangle has anywhere between 6- 10 pairs. This time of year look for berry bushes and you will find the mockers. There is a reliable pair at the Oshawa train station just east of the building along the Via platform. Also there are pairs in the lynde shore area, south ajax, pickering nuke, etc.
Research done has shown that Durham regions mockers are doing well and are increasing. Would not surprise me that one has not yet shown interest in the scrubby forests se of Orono that are filled with berries most winters.