So we know that Pooja wrote some incriminating entries in her diary and it's obviously enough of a threat to Mr. Hooda to hire people to find it. Arjun had it in his possession for all of a minute before it was taken from his drawer and then yesterday he saw his dad leaving his room and then discovered that the diary was back in his drawer again but with pages missing.
So why return the diary at all in the first place? We can safely assume that Mr. Hooda removed all incriminating pages from it so then why return it? Why not just burn the book? By returning the diary, Arjun now knows that there are pages missing and that his dad is the likely suspect since he was seen leaving his room. It's just going to make Arjun suspicious of his dad. He did tell his dad the other day that his dad was the expert at covering things up so it's probably just a matter of time before he reads the diary and figures out some of the clues. Or he'll approach Adi with something he read and then Adi will take the diary and start to put pieces together.
Anyway, just a few thoughts going around in my head!