As the other comment said above (noot noot), he can't hide the poison more than he already did (the paper packets). I think his plan was just fine. It's just that he didn't close the cabinet properly, if that was the only storage space he had that the bottle would fit. It was smart but unfortunately he was careless.
His plan was fine lol just unlucky
Then his plan wasn't fine. He had already made so much preparations, and yet he would forget to close the cabinet?
Even if you're not hiding something, it's honestly an instinct to close the cabinets. I get that the author wants to use the servant boy as a catalyst, but the execution is so sloppy that it's stupid. They also made Yeonjo act out of character, that's its unconvincing.
For a smartass who went through a lot planning everything, and even threatened an official to join forces with him killing the governor, Yeonjo was dumb this time, putting the poison somewhere easy to see. He could've carried it with him all the time, or buried it in the soil, idk. Seems like the author is running out of ideas for the angst. Also, he could've just run out to get the antidote himself, without saying anything suspicious to the guards. When he comes back, he could just say he was overly worried so he lost himself. I'm sure rhe governor wouldn't mind.