If I genuinely believed I had somehow inadvertently been present at an (extremely) unfortunate situation in which 2 people I didn't know died in 2 separate 'accidents' in 2 separate ways within minutes of one another, then yes, I'd definitely panic. I'd probably run around in circles for a hot minute, go completely blank, lose control of most bodily functions and wonder what the heck just happened. But then I'd get myself together and call 000 and wait at the scene til they arrived. Then I'd call my partner. I'd be absolutely shattered. I'd be in therapy til the end of time.
I would absolutely not remain cool, calm and detached, meticulously clean up the crime scene, return later to destroy all evidence that may have served to show my 'innocence', repaint my car with house paint, remove the awning that the family remarked was just like mine in the 60 Minutes program and worry about losing my knitting club membership.