I agree that deadly poisons are unlikely to be found in the hands of a regular bandit.
But given a bit experience in robbery the bandit will have faced the proximity of death,
meaning he wants some "insurance",
something helping him in case things get unfavorable,
in a sense that someone scratched with his weapon (prefered) an arrow is unlikely to be able to follow him.
So what to do,? Fighting skills are not the best, equipment is bad, luck (Phex) is not always on your side.
So he might remember something like:
His parents telling about never to eat the following plant,
or just to eat some parts of a plant properly prepared,
the bandit might have also experimented with some plants effects when he was young,
or eaten somthing wrong due to hunger and survived suffering,
or be aware of some poison which is used on a regular base to poison animals,
- for example as in reality fly agaric was once used, if I remember correctly, cooked out in milk to catch and kill flies.
So the bandit may definetely be aware of poisonous plants and mushrooms and
depending on the background trying to extract or concentrate the poison by cooking might seem as a resonable idea to try.
The effectiveness of the such created poison might be checked on an animal.
Depending on the result of the check and the character of the bandit he might then poison his weapon before an ambush .
It may be far less effective than the product of an Alchemist, unlikely to kill someone
(which is usually not intended anyway, since many of them are interested in robbing not killing),
but might suffice to take the victim (at least partially) out of combat and bind in addition forces that try to help victim.
So I would conclude the regular bandit has in principle the knowledge and the means to create something poisonous,
if he considers that option or not depends on character and circumstances, if the created poison would work as intended, depends on luck and background,
so I would think of a 5 to 10% probability of poison use and going back to the original question as a GM
I let bandits at some time (randomly with low probability, trying to be fair but not predictable)
use a poison if it seems reasonable.