OoC: If the Seer investigates a person during the day, and he finds out the target is innocent, he notes down the name. Thus, if he dies, the other prisoners learn about persons who were investigated and found innocent.
If he finds out the target is an assassin, he doesn’t write down the name, he has to tell the other prisoners (if he chooses), otherwise they will not learn about it.
The note found in Nghtstalker’s cell is not addressed to bugcat, it has bugcat’s name on it. A guard found it so the announcement can be made, the prisoners are alerted by the voices and find it where the guard left it, without making much of it (he had to play dice), in the wall crack. The rest of the text is something like “I followed Marco and he didn’t do anything suspicious”.
tl;dr NghtStalker was the Seer; bugcat was checked and found innocent.