I can only speak for how I Know I would have behaved in that situation. First, I'd be horrified and frightened. I realize people are different, but having been in a similar situation (no murder, no arson but crime scene taped off), I went to the first uniformed person who looked like they'd be the nicest person or least likely to bite my head off (some really look hardass just standing there and that intimidates me). Everything was fine for me. In this case, I would approach an officer who is not occupied or busy and tell them my of my relationship to the homeowner and what happened that morning because I had been there. The officer would contact his superior or the watch commander who would then determine whether to take my contact information and brief story and let me go or to have me wait there and be taken to the station for a more in-depth interview.
But, that's me. First, it would never occur to me to leave without saying anything to anyone because it's an extraordinary circumstance and I was involved however innocently. Secondly, I'd be a freaking nervous guilty-feeling wreck waiting for someone from LE to discover I'd been there for an errand in the morning, visited the scene during the fire and LE activity and said nothing because I KNOW at some point, they'd know I had been there and want to know why I didn't tell anyone.
I realize that goes against advice of volunteer nothing without benefit of legal representation, but I'd still do it. I can't handle suspense of any sort. I'd want to unburden as soon as possible and deal with what happens as it happens. For me, that's infinitely less stressful than waiting for that shoe to drop.
I would do this because it makes sense to me. I don't care if I didn't know a murder had been committed, I'd know that I was given a task by my boss to do something unusual and I did it. A few hours later, I'm informed that the boss's home is on fire. I go there to see what's going on or if I can help in some way. I then see or hear about bodies being taken out of the house. Hell yeah, I'd talk to an officer there.
I'd talk to an officer if there were no bodies being removed if it was just a simple fire. "Officer: what happened? I work for Mr. S and ran an errand for him this morning to drop something off at his house. I just got a call that his house was on fire and came to see if things were okay." And then, things would proceed from there.
And I absolutely don't believe that his car was there "for hours" as stated in that article that deliberately smushed together information to make it appear to be something it wasn't. It could have been a matter of minutes when he arrived, got out of the car to get closer on foot, came back to the car and it was taped off. At that time, police didn't know it wasn't a neighbor's car - it was just there.