If such a clean-up leaves even the tiniest trace and the investigators are worth their salt, this will land people in jail for obstruction of justice. Doubt anyone wants to risk jail time in their already pretty screwed situation.
I might be giving CA more credit than they're due in the smarts department, but considering that I'm just a dude making up scenarios, I find this particular one pretty plausible. And assuming the worst seems like a sensible approach in this case.