Poisoning a water supply is actually quite difficult, unless you can access water after it leaves a water filtration plant, and whatever poison you use is not particularly susceptible to chlorine or sodium.
Unless the poison is a hardy bacteria, like E. coli, or other coliform bacteria found in feces, *and in sufficient concentration* by the time it reaches consumers, it is unlikely to cause illness.
Probably the poisons that would set off alarm bells most are rather hard to get.
But your conclusion is disturbing:
Probably the poisons that would set off alarm bells most are rather hard to get.
"Hard to get" in the sense of being expensive? Well, somewhere, someone with a lot of _____ money would probably be willing to fund a "research" project. A well-coordinated, large scale effort could prove to be a cost-effective way to eliminate one's enemies.
Secondly, if the contaminates are introduced at the user side of the filtration, how much damage will occur before any such alarms are activated? (Rhetorical)
Hope our people are all over this.