If a test has a time limit and the learner exits the test before submitting it, the timer still runs in the background. When the learners return to the unit of the test, the system takes them to the last unanswered question and the remaining time of the test depends on how long they were away. If the time runs out and the learner hasn't returned to the test unit, the test is automatically submitted with the answers answered up to that point.
If you want to ensure that your learners cannot go back and change their answers after exiting the test and re-entering, make sure the first box in the screenshot you sent us is unchecked ("Allow movement to next/previous question").
Unfortunately, leaving the test unit without submitting their answers is something that cannot be traced.
However, you can allow your learner multiple attempts for a test if they fail the first time. Just go to the Test options and check the box as shown below:
