A non-senior player that doesn't see any playing time in the whole season will receive a redshirt, if they haven't already received one.
I noticed in one season, my 3rd- and 4th-string quarterbacks were automatically granted a redshirt because I didn't use either during the season. The 4th-stringer was a freshman, and the 3rd-stringer was a sophomore I had only played during his freshman year.
A senior not playing an entire season still graduates and leaves the program. I once tried redshirting a senior and, even though he had an extra year of eligibility, he still graduated and left.