Can you season skewers after cooking them?
It's very common to season steaks or other meats with salt and pepper after cooking, there would be nothing unusual about that. There are also seasoned salt mixes which people may use as well, that's also common. Using other spices is less common, there are finishing spice mixes which could be sprinkled on, which is less common.
As a writer myself I would say it will work any way you want it to. I would think nothing of seeing someone sprinkle some salt on food after cooking, unless I had reasons to be suspicious to begin with, and then I would touch and smell. Or I might not have suspicions until I smell the food and detect something odd.