How should a student handle possible misconduct in his supervisor's work?
Misconduct is of course a very serious thing. The problem you face is that you do not know the circumstances when your advisors work (or lack thereof) was made. It will also be hard to start digging into it to find out so I am afraid there is probably not much you can do. I do not think it would be wise to start spending time on this as a student either since it could seriously affect your thesis by distracting you and waste your time.

So what can be done? I would try to bring the question up with others. Discussing the different works and see if others pick up on the problem. Making people aware of what might be a problem can help you further assess the issue. I would tread carefully, however, unless you feel secure in opening up a can of worms. 

I fully realize that this is far from a satisfactory answer from a moral point of view but it is pragmatic and I do not think it is your burden as a student to carry such weight. So the best you can do is to carefully expose the issue and perhaps later do something about it, if you still feel it is a serious breach.