Complications from Mary's birth would claim the life of her mother on 5 September 1548, and her father was executed less than a year later for treason against her cousin Edward VI.
[1] After her second birthday in 1550, Mary disappears from historical record completely, and no claim was ever made on her father's meagre estate, leading to the conclusion that she did not live past the age of two.
[2] Victorian author Agnes Strickland claimed, in her biography of Catherine Parr, that Mary Seymour did survive to adulthood, and in fact married Sir Edward Bushel, a member of the household of Anne of Denmark, wife of James VI and I. Strickland's theory suggested that the Dowager Duchess of Suffolk, after her marriage to Richard Bertie in 1553, and before she fled England during the Marian Persecutions in or after 1555, arranged Mary's marriage to Bushel.
Porter further speculates that Mary is buried in Lincolnshire, near Grimsthorpe, the estate owned by the Duchess of Suffolk, "where she had lived as an unwelcome burden for most of her short, sad life.
A similar premise allows Seymour's supernatural powers to help her friend Alison Bannister search for her lost child in the 2016 novel The Phantom Tree by Nicola Cornick.