By rumors, reason of absence of human prisoners is probably that they tried to escape but they were caught by Basilisk and turned into the stone. Again I would like to point that it is only a rumor. Clues that could confirm that are there were camps in Dwacatra and each representing group of monsters - but one camp is just without humans, and instead we can see trolls (they are not mentioned in the book). If we will take a look on map, we will see Basilisk lair, and and the other side we can find some Knight Statues, which can represent human knights escaped from prison, but turned on stone by Basilisk.
Other theory can by just reference to prisoners who have escaped from real Prison of Alcatraz. At their camp we can find trolls - real prisoners used handmade dummies; real prisoners were digging hole to escape - in Dwacatra we can suppose prisoners did the same while we can look at map layout; real prisoners or their bodies were never found, so same in tibia, even while we can see Knight Statues, we can't connect them in 100% to the prisoners.
So, answering on your question - I dont know where are those people, but I hope one day somebody will find answer.
