Teensy Tabby is Sarah's lab note 6 of 6, a short audio log she records on Wizard Island. You get it automatically during the story mission This Could Be It, so it can't be missed.
Sarah's first serum test kills the young Newt she had followed for months, a girl in a Teensy Tabby shirt. She is relieved Deacon knows she isn't building a weapon. The note cuts off as Colonel Garret's men seize her.
It leads straight into Sarah's capture by the Militia.