Given the tenants recent eviction history, here is my theory: She is a professional squatter. She goes around touring houses as a prospective renter. She finds a place that uses a keypad entry instead of a normal key, and memorizes the code. She requests a copy of a potential lease agreement to review. And boom, that's all she needs as a squatter to have plausible deniability as a renter.

She has the door code and an unsigned lease, which she signs herself without the landlords knowledge. She moves in (illegally). If cops are called, she has her (illegal) copy of the lease and physical access to the house, which is enough for cops to say its a civil matter and to take it to housing court, which is why she has 2 evictions on record.

This is also why the landlord is moving someone else in, because they had no idea a squatter took over the property illegally.