Where a autocomplete element is used outside of a Moodle form, it is not always detected as being an autocomplete by the behat field manager.

This means that you instead up with an additional set of steps like:
I press key "13" in the field "foo"

The reason it is not detected as an autocomplete is that the detection comes from the parent div created by the form template.

The autocomplete code can easily add a new data attribute to more readily identify itself as an autocomplete to give a more consistent use.