We were glad that she had made a friend. we weren’t sure about the offhand attitude in which she ditched the presentation and us! besides, where was this trail, and just how safe was it?
which word or phrase provides a transition between the second and third sentences?
just how safe
besides
where was
and us!

Besides

Explanation:

A word or phrase whose function is to provide a transition between two or more sentences, paragraphs or ideas in order to help make ideas flow smoothly are called "Transitions words". Depending on the transition word used, they may show similarity, illustrate a point, give examples, express contrast between one idea and another, add information to a previous point, among others.

The word "besides" is a transition word because it provides a transition between the second and third sentences by adding more information to the second sentence. Other transition words with the same function are moreover and furthermore.

from the excerpt from the "the dark game" the reader can infer that folke cronholm was spying for the allied nations.