Unfortunately it's difficult to guarantee discretion - for all you know the person you're contacting is the brother of the candidate's boss's girlfriend, for in

Unfortunately it's difficult to guarantee discretion - for all you know the person you're contacting is the brother of the candidate's boss's girlfriend, for instance. Furthermore, in my experience, references like to know in advance that they will be contacted: in my culture, any ethical reference is going to want to know who's asking, why, and whether they have permission to speak on the candidate's behalf. Finally, there may be personal issues between the person you're contacting and the candidate that would make the reference biased. Ultimately it seems pretty straightforward and professional to simply ask the candidate outright whether you can contact X, Y and Z as additional references (unless you happen to know these people well enough that discretion is relatively certain). At that point the candidate can say, "sure, but let me speak to them first to see if they're OK with that", or perhaps "No, Bob stole my wife from me and I hate that guy."