It's just like in medicine. You only test for what you know to test for. Hence the number of people with Hepatitis C and HIV from having received blood products.

What I would like to know is how such a chemical managed to make it into the food to begin with. It's not the type of thing you use to kill rats in North America, at least from everything that I've read.I know it's not in Canada. 

My reasoning says that someone disgruntled employee put it into the product.

Menu Foods seem to be doing all the right things in this situation. They aren't hiding behind their lawyers. They are taking full ownership of the situation and are paying all medical costs for those animals affected.  The company is taking a huge financial loss, because even safe products have been pulled off of the shelf.

If it was product tampering, and I think it is, I hope they catch the person and prosecute to the fullest extent of the law.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi