The gemara teaches us that nowadays one should not warn one's wife not to seclude with a man since this would assur her forever if she had yichud with him as there is no longer the water test.
I heard from a Rav that if one would innocently warn ones wife against having yichud with a certian man one is in violation of this gemara.
Surely if one warns ones wife purely to stop her transgressing the issur of yichud(eg be careful not to have yichud with this guy)and the woman accidently had yichud with him it would not cause a problem.
I would have thought it would only cause a problem if you suspected a certain man of something specific.The Torah seems to imply that the catalyst to the kinnui is seeing something improper between a certain man and his wife.If you do not say this it is a crazy chumra whereby someone would be trying to help his wife not to transgress yichud and it could result in him never being allowed to live with her.
Furthermore I saw a halacha whereby if the kinnui would be a warning not to cuddle it would not be a kinnui. It needs to be a kinui for biah. The Aruch Hashulchan says a stam kinnnui is for biah.Maybe in this case one could say that in such a case where it is clear the husband did not have in mind biah(he only wanted to save her from issur yichud)it would not be a valid kinnnui.
What do you think
shmaya ormonde, london
Rav Moshe Sternbuch writes in a Teshuvah (Teshuvos v'Hanhagos, Vol. 3 #410) (and he told me personally) what you said: even though Kinui may not require two witnesses, but it has to be a warning that is motivated by the man's suspicion of his wife's motives. Simply telling her that Yichud with Ploni is forbidden in order to prevent her from violating the Isur of Yichud does not constitute Kinui.
Kol Tuv,
Yonasan Sigler
This is not a Psak Halachah