The NCAA Division 1 rules have been updated to include the 1992 Interpretation:
14.2 Full-Time Enrollment - 14.2.1 Requirement for Practice:
4.2.1.7 Exception—Former Student Participating in Practice on an Occasional Basis—Sports Other Than Football. In sports other than football, a former student (e.g.,former student-athlete) at the certifying institution may participate in an organized practice session on an occasional basis, provided the institution does not publicize the participation of the former student at any time before the practice session. (Adopted: 3/3/11, Revised: 4/25/18 effective 8/1/18)
This appears to be used by top level swimmers in particular.
I didn't check the DII rules, but the DIII manual doesn't cover the issue at all. I know of at least one non-student runner who was allowed to participate in DIII practices after introducing himself to the coach and signing a waiver.