I know nothing happened between noina and mihir that night and it is proved.
But wait my issue with mihir was not that he spent night with noina or not as him being a red flag and that too after so many years of marriage was hinted many times during his so called friendship with noina even when he was married to tulsi.
He crossed boundaries with noina which a married man should never cross and i am not talking about just physical boundaries here. He trusted and admired noina more then tulsi during some tracks and when his leg was injured while being still married to tulsi he actually thought noina is always going to be there for him rather than tulsi his better half.
Even after this leap and mihir crying few tears for tulsi he still has to do lot of redemption which is missing. After revelation of no one night stand with noina he feels as if he never wronged tulsi for noina which is so not true. Mihir may not love noina but for sure his actions and choices were questionable before tulsi left shantiniketan.
Even after so many years of marriage with tulsi and seeing her love and sacrifices for him and his family he still didn't trust tulsi enough to have faith about her decision being correct on pari and angad during their marriage tracks.
He is now shown fully relaxed and guilt free that he didn't touch noina that night but what about emotional distance and emotional cheating he did? What still makes him red flag is he still can't see his mistakes though he says tulsi is right in not coming back. Dude if you really felt that then you would not force her.
Now what to say on tulsi as i was her fan till this new noina darling version two promo where our smart tulsi will get noina married to mihir as her last wish 
. Cvs are hell bent on spoiling mihir more and now they are on track to spoil tulsi's character too.
Fed up of noina now. So many things still are left to be shown. Even tulsi mihir relationship can be shown slowly progressing if tulsi decides to forgive mihir which she eventually will.
Just my opinion on kyunki as i loved it more after tulsi left shantiniketan and even was excited for her comeback in shantiniketan until i saw this noina promo. 
Arjun
Anupamaa
4