Can a recruiter contact a postdoc's recent supervisor who was not listed as a reference on the application? P, a postdoc applicant, lists 3 referees (A, B and C) on her strong application.


A knows P's work best and gives a mixed (guarded) assessment.
B knows P's work moderately well and gives a great assessment.
C knows P's work least and gives a great assessment.
S and T, a recent supervisor and a recent employer, are not listed as referees.


Can the recruiter contact S and T for further feedback? On the one hand, they are not listed as referees, but on the other hand, the applicant has not requested not to contact these people.
At least in US academic culture, the fact that P is applying for a given position, or is applying for jobs at all, should be considered confidential.  As such, if you're the recruiter, you should not contact S or T or anyone else regarding P's candidacy, unless you first get permission from P.  (Obviously, you have implicit permission to contact A, B, C, since they were listed on the application.)

Asking S or T for a general assessment of P's abilities, without saying that they're applying for a job, is a gray area.  I would advise against it, since people will tend to infer that your reason for asking is because P is a job candidate.

If you're the candidate, on the other hand, you should be prepared for the possibility that the employer might contact S or T anyway, even though they're not supposed to.

Expectations might be different in different cultures.