Well, that would certainly explain why I couldn't find her! As for 1900, the surname is spelled Tetsloff.Note that the pronunciation of that last name would have an "f" sound at the end, whether spelled with a "w" or "v", so the above spelling would be consistent with the pronunciation.
1900 US Census Ward 6, Pittsburgh, Allegheny Co., Pennsylvania Bluff Street
Tetsloff, Mary, servant, born February 1885, aged 15, single, born Germany, parents b. same, immigrated 1888
1910 US Census Roselia Foundling Asylum, Cliff & Mariella Streets Pittsburg, Allegheny Co., Pennsylvania
Tetsloff, Mary, lodger, aged 22, single, born Pennsylvania, parents b. Germany, maid, hospital
I think that the 1910 census is definitely yours; perhaps not the 1900.