Personally, I think detectives calling KOS on someone who calls them out, usually as a joke, is just an excuse to RDM, and on many servers where "use common sense" is in effect, it wouldn't be a killable offense. It would make sense if you could even disguise as the detectives, but you can't*. This situation isn't explained explicitly in the rules as far as I can see, so it being more clear one way or the other would be great. *I don't know if there's an advanced disguiser weapon on the non vanilla servers, but this is specifically for vanilla.
I've seen traitors mistakenly KOS a detective when they were just looking to KOS someone randomly to get them killed, as traitors will often do. It's by that logic that I determine whether or not to kill them for KOSing a detective. KOSing someone who is proven or a detective can automatically be processed as a false KOS, which is traitorous.
No one is going to follow through a KOS on a detective though. What traitor would do that on purpose.
A KOS is a KOS and if you think about it, sometimes if a detective is mass RDMing, someone may think to put a KOS on them, even though they just KOS'd a proven innocent. Common sense, like you mention, would be to not call a KOS on detectives.
Common sense says you'd give some kind of indication that a detective was mass rdming other htan calling him out, because with no further communication no one is going to follow it.
Traitorous act: Calling KOS on innocents. Players know that Detectives are, by default, innocent This implies that calling KOS on a detective is inherently traitorous You can be killed for traitorous acts
If you call someone a traitor, that's a KOS. If you say "XXXX [Non-detective] is doing <insert traitorous act here>" that's also a KOS. Note the non-detective part.
