what is the verb in past tense in this phrase?
Duo will give you quite a bit of practice with 'atrás' - I guess as one of its translations is 'ago' - which is a link with things past.
Duo does make mistakes - albeit very infrequently. Clearly, as you say, this sentence is phrased in the present.
Perhaps it refers to a tense past instead; that could be why she is hiding behind the door now.
Why is it wrong for "the desk is behind the door"?
Because the phrase refer to a woman that works at the office. A secretary. Not a table.
But "secretária" also means "desk", so "The desk is behind the door" is also correct.
... com a faca que atingirá a parede em poucos segundos.
Why esta and not e
Because a woman is a movable object i. e. she can go somewhere else.