Go straight to an upper level housing manager. When I moved out of my house 4 months ago, I was told that blacklighting to look for evidence of urine was now "standard procedure" for every move-out. That should have been identified and the last tenants should have been charged for the carpet. If they're too lazy to follow their own procedures, you have every right to make sure it's done NOW so you're not charged in the future.
Also...I got a cute little blacklight at Petco for about $7...it's a keychain light with a solar charging panel. You have to wait til night-time to use it, because it's not THAT strong, but mine is strong enough that I spotted some accidents from my pet when it was completely dark in the room. (Time to start saving money to replace THAT carpet. LOL!)
Anyway, get yourself a cheap light and check for the spots first, and then you have ammunition to call housing and demand that the damaged stuff be replaced. Whether they didn't investigate upon the last tenants' move-out, or they did, and they charged the tenants for a replacement that never happened...they screwed up either way.