At first I thought it was Raunak, because I saw a quick glimpse of the stalker as he was getting away from Drone. The eyes and hairstyle looked a lot like Raunak. However Raunak is in the mental asylum and what would Raunak have to do with Suraj Singh.
Then it struck me that what if Arya is Suraj Singh's son and he's the one who is either stalking or getting someone to stalk Drone, while he is with Simran.
But then again it could be Raunak as he may have found out that Simran is alive and therefore cheated him and the family supporting her. So therefore he wants to get his revenge against the family.
I may be wrong. What do you think guys??????