Yes, AOs will sometimes contact an applicant’s GC if they have questions that need clarification. The reasons for clarification can run the gamut from how the applicant compares to other students in the HS, get mid-semester or quarter grades, ask about something the applicant, a teacher, or the GC wrote in application materials, or anything else one might imagine.
I don’t know how often this happens, I expect some schools do it more than others, and AOs that have a relationship with a particular GC/HS probably make use of that relatively more as well.