Random individual deaths are often prisoners with vulnerable reputations that have been assassinated. The "mouth and ear" icon indicates a snitch and the "broken badge" icon signifies an ex-cop or ex-prison guard. You can check the corpses as they leave in the hearse (or in the morgue if you have one) to see if this is the problem.
The solution is to screen all new prisoners as they arrive, transfer the vulnerable ones into the Protective Custody category and then put them in permanent cell lockdown and/or build a separate secure wing for the PC category.
Activating an informant will reveal hidden reps and any prisoners that are currently marked for assassination, but only an informant with 100% coverage will have info on the entire population of the jail.
