Taking your story as verbatim there must be an underlining reason for your department manager acting in the way they have. From what you say they were party to your employment, do you know if they objected to your employment and were overruled, as this may be the reason they act in this way, I suggest you consider all the possibilities for this person acting in this way and try to establish why they do so.
Then speak to them over your mutual concerns, involve the manager if you believe it will help, there is a lot that can be achieved by honest discussion
Under employment law there are implied duties of mutual trust and confidence, if your personal possessions have been interfered with by your employer without your consent you are correct to raise a grievance, but it seems your employer has investigated your claim but is unable to satisfy themselves that it occurred due to a lack of witnesses. Remember your employer must be fair to your department head as well as to you.