Here's what I have from the related thread where I first found out Maria existed
http://www.rootschat.com/forum/index.php?topic=598200.msg4481100#msg4481100and follow-up research.
1841 (with mother Mary and her family)
Address: Old Town Thornhill
WOODRIFF William M 60 Ag Lab England
WOODRIFF Janet F 60 Perth
WOODRIFF Mary F 20 Perth
WOODRIFF Jane F 7 Perth
MONTEATH Maria F 1 Perth 1851: High Street, Thornhill, Kincardine, Perthshire (with married mum and stepdad)
Head: John STEWART 33, Tanner
Wife: Mary 34,
Dau: Mary 3,
Son: James 8mth?
Stepdaughter: Maria MONTEATH, Scholar,ALL stated born Kincardine
Then I searched but simply couldn't find a marriage or death, no further trace
UNTIL TODAY 
On familysearch I came across this entry and I think it's the birth of her daughter.
Name: Mary Henderson
Gender: Female
Birth Date: 06 Dec 1862
Birthplace: KINCARDINE BY DOUNE,PERTH,SCOTLAND
Father's Name: Alexander Henderson
Mother's Name: Maria Monteith This may be Alexander
Easter Daldorn, Norriestown Parish Kincardine - Perth
HENDERSON Alexander M 15 Agricultural Labourer Born Outside Census County
(1841) That would make Alexander around 15 years older than Maria.
What I cannot find is a marriage for either of them, nor on any other census material. Where did they go?
My question is ........... I feel a song coming on
How Do You Solve A Problem Like Maria? 
Can anyone help?