I gave up looking for gender clues a third of the way into the first book; it's clearly very deliberately undefined. Even after the crew has seen Murderbot's face they don't slip into any masculine or feminine pronouns, which means either they're all really diligent about not assuming gender (it is the future, after all) or its human features are so perfectly androgynous that it doesn't read as either.
For better or worse, this is a reason Murderbot comes across as female, based on conventions of the genre and societal expectations. Similarly, it’s what one assumes when they find a Sci Fi author with two first initials and a last name, or a forum poster with a gender-neutral username and an anime avatar..
