So what might such a not-readily-identifiable means of death be?
As for the forensic evidence, if it's not immediately obvious that wouldn't be a problem for my story, since we're following it through a curious coworker who wouldn't know everything the police know.
This answer may sound flippant, but not meant to be. A not readily identifiable death would be any death that doesn't seem to have a readily identifiable cause. Hanging - that would be a readily identifiable cause of death (the strangulation/broken neck/etc common with such things). This would give the police (or more correctly the detectives) something to focus on and that would wind up either being ruled homicide or suicide very quickly.
But, if your character goes backstage, finds the dead guy lying there and rushes to his aid...only to find the face purpling and the skin cold to the touch and realizes that THIS GUY IS JUST DEAD! No hanging, no serious wounds, etc. Then that causes it to become a longer investigation while they determine the cause of death and then begin to look for the explanations for it.
Some examples of what could have been like this is: he's taking a medication that could have a toxic buildup in the body such as lithium or warbutrin. During the autopsy the toxicology reports come back that there lethal levels of this medication in the person's system and that is most likely the contributing factor for death. So, the police go to investigate and someone remembers that they did see a prescription bottle of the medication (for example: warbutrin) in the area where the guy was found. They investigate and they determine that either the prescription as filled awhile ago and yet the prescribed amount is still mainly full. Or that the prescription is relatively recent but the bottle is empty.
Which could lead to the idea of suicide, except someone else had mentioned how lethal a build up fo this drug could be to the character. Someone who didn't like the character and is just shady enough that others may believe he would kill someone. Or they find traces of the drug in the guys lunch or drink. Which looks suspicious. (only to find out that maybe the character had problems swallowing pills so he would crush them up and put them in his food.)
Have him cut his throat.
Believe it or not, many people who attempt suicide this way survive. The tendency is to tilt the head back but that pushes the windpipe forward while the carotid arteries are protected by major tendons. They just end up cutting their windpipe.
It would be bloody and the victim would not be able to talk when found. (This may or may not work for your situation.) But it could very well look like a murder attempt.
Once his windpipe is repaired, he'd be able to talk (or maybe just whisper?) The repair itself is usually straightforward.
Yes, but in this instance the doctor would give the medical opinion that this was a self inflicted wound. The angles, force and other indicia from the wound would be readily apparent that this was self-inflicted and not inflicted.
Rabe...