Lots of poisons can only be found if someone is actively looking.
The castor oil plant seeds can be used to make ricin pretty easily. I just looked it up; it grows anywhere tropical/subtropical.
The symptoms look like typical food poisoning/travels’ stomach.
Just looked; ricin is also basically impossible to test for. They have to look for anti-bodies.
I wouldn’t be surprised if the DI has a serial killer on the loose.
I am familiar with ricin thanks to Breaking Bad. It produces flu like symptoms that cannot be traced. No mini bar for me on vacation.