For me, doing nothing is not an option. When I see stealing under my nose I am not going to ignore it. Again, I suggest the HR route. This should not get back to you as they are bound to protect your identity. Is there a suggestion box or some other way of submitting an anonymous report? If none of these are an option, I would still go to someone of higher authority. You need not even identify the individuals in question rather simply state that this is a problem that exists. They ought to then start an enquiry as a result.

I wish you luck in resolving this.