The Iranians tend to dispense with the need for prisons, either through such judicial practices such as stoning, or by ensuring that prison survivability is low.
Besides which there are such things as undocumented prisoners and a lack of habeus corpus.
But that isn't the issue is it? Iran is a country which uses children as human waves to clear minefields. As was said earlier, life is cheap.
Plenty of people seemed to disappear after the pro democracy demos following Dinnerjackets last election, whilst a lot of people such as Mousavi and Karroubi have been held without trial under house arrest. Even Europeans are not immune from disappearing without their embassy's being informed as happened to a former work colleague.
The vast majority of the conscript army are no better than cannon fodder, just like they were during the Iran Iraq war. My brother in law had to spend two years as a conscript after getting his engineering degree. Sickness was rampant in his barracks whilst conscripts only had one change of clothing per week. After basic training was over many of the conscripts went on to guard museums and banks, whilst he went on to do basic admin tasks in an office. That doesn't mean to say that the Revolutionary Guards etc are in the same boat though.