Could he have left through the window? Locked the door from the inside then left through the window. This scenario would almost look like he was fleeing/hiding from someone. Wanted them to think he's home when he's not. Were the lights left on or TV or something?
Too farfetched?
I'm thinking he has/had a secret. Maybe a secret relationship or something to do with drugs or money. Maybe he was blackmailed or threatened. Maybe someone put a lot of pressure on him in the days before which is why he sounded confused - too preoccupied with the severe problem he found himself in to talk about the usual stuff with family?
Maybe that person demanded to meet with him and he thought he'll be right back but things turned nefarious.
Regarding the question whether he called in sick or he left work sick. Could he have called his boss while at work, told him that he doesn't feel well and will go home? Maybe boss was in another part of the building, or TC didn't want to face him, so he just called him and left. That would explain why both was reported.
I don't think he was sick, I think something was going on that he had to take care of urgently, right then.
All jmo moo.