Any staff member at a Class V leaving whose contract is about up does undergo a sec check if all points are in. They are done by a staff auditor. It would piss off the Qual Sec, because he wanted the auditors auditing paying public, as that was his important statistic. There are supposed to be staff-staff auditors who conduct auditing actions on staff, who would handle this and other actions on staff. Haven't seen a lot of orgs that really had that and so the 'regular' auditors would have to be used, which meant staff waited. I've seen staff who had a way to get money (their spouse or whatever) actually pay for their routing-off sec check so they could get out of there sooner.
I did a few of these, and it SUCKED. There was vicious pressure on the auditor to pull the right withholds so the staff member would stay. I got threatened quite a few times with an SP declare. I'd be shown the references where the only reason a person wants to leave is because of a missed withhold. Missing a withhold was a High Crime, i.e. Suppressive Act; therefore if the staff member still wanted to leave, then the auditor had really messed up
. My worksheets and tapes were reviewed constantly, with input from the ED. The C/S would try to keep adding more and more questions to get the person to cog and stay. Freaking nightmare.
The other sec checks given while I was there was for people in HCO being posted. Someone who goes into HCO is supposed to receive a sec check. I haven't always seen this done, but there was policy, and it came from uplines that this needed to start happening.
Contrary to what you hear, what I saw, it was rarely that staff members went through sec checks. They had to be valuable--up in the org board or making money (such as a reg) or on some other post really important for the more important stats; and then fuck it up, and the sec checks went crazy to handle the scene. More often, the person would be stuck in the ethics area continually writing up the O/Ws and doing lower conditions and very often MEST work, until they 'popped out of it'. They were also pretty shunned by the rest of the staff.
As far as who does the sec checks, there aren't really many SO members hanging out at Class V orgs. Those that are are on the admin side, and so wouldn't be auditing anyone, let alone staff. There are 2 real exceptions. When an org reaches "St. Hill size", they get a team from the "Universe Corp" (composed of SO members), whose job it is to audit the staff up through the OT levels.
The second is when an org really fucks up and a mission comes in to sort stuff out. These come in with the intent to handle ethics, and boy did that suck. Mostly, ethics interviews and rollback and conditions, with some folks getting sec checked... very nasty business. You'd walk into the org, and see all these staff members scrubbing the walls with toothbrushes. Seriously.
ETA (just because I continue to see the misconception): Staff members are not put on the RPF. That is strictly for SO, but they do have ways of abusing the lowly folks, too.