I have a question.
Can all the professors in a department see a particular student's grades?
Can the chair of a department see a student's grades?
Another variant of the various models described in these answers, to illustrate the vast diversity in procedures and how there is no globally correct single answer to the question:
I am most used to exam corrections being community efforts. That is, when a (written) exam of a class by one professor has taken place, the whole department of that professor will be asked to help checking the exams. This is so the (substantial!) workload of correcting 50+ exams is shared.
In effect, it usually means that a considerable part of the PhD candidates of the department will do the checking, and often (as one person checking one particular task across all students is more efficient than one person checking all tasks only for a small set of students) this means that each of these PhD candidates will have seen the overall performance of each student in the exam.
Summary: In some places, it can be assumed that everyone within one department knows about student grades in some way obtained from that same department.
As a rough guideline, this can easily mean some 20 people or more were involved.