Do universities keep applications on file after the current admission season has closed? Say you applied as an undergrad and were denied. Then re-applied for grad school. Would a school check to see if you had applied previously? Would it make a difference if you had been accepted but then declined the spot?
Grad admissions is separate from undergrad. Don’t worry about this.
Undergrad and grad schools generally have separate admissions processes – the former is handled by Offices of Admission, the latter is handled by faculty in individual departments. The institutions themselves might have knowledge of an undergraduate application’s rejection or admission, but it should have no bearing on grad school admissions.