Some people are thinking that Yash hired the detective to follow Aarti, well I am not so sure about that. Yash has always been open with her regarding what he thinks or feels. Few instances that I could think of-
1- On their SR he told her what he expected from this marriage.
2-when he changed Ansh's school he explained the reason why.
3- during Palak's birthday when Aarti tried to imitate Arpita, he shouted at her in front of the whole family and guests.
4- whenever Aarti( in the initial stage of their marriage) raised the topic of raising children he openly opposed her.
5- when she forced him for the Mumbai trip, he sternly said no and gave her the reason why he can't go. He even told her not to interfere.
6- After returning from Mumbai he told her and the whole family that she no longer is his wife.
7- when Aarti confessed her love he rejected it.
8- when she Aarti had tied four coins instead of three on her saree Pallu during some Pooja and Yash noticed it. He simply dragged her in their room and told her that moms wish will never come true.
9- when Aarti ran away with Ansh and he brought her back he publicly said that now he has accepted her as his wife.
( I know there are many such instances, but these are all I could come up with)
So what I am trying to say that Yash has always been open and frank with her regarding how he feels about her. If he will suspect Aarti is doing anything wrong or is having an affair with P, I don't think that he will hire a detective for that. After considering all the above facts Yash will directly confront Aarti instead of going behind her back.
Well that's my POV regarding Yash's nature. Please comment on what you think.