In Chapter 80 Mimasaka was already inside Soma's room waiting for him, to the horror of the other residents (and Ikumi, who was also visiting), showing that he could freely get anywhere he wants. Doubles as a mini-Crowning Moment Of Awesome for Soma and his Nerves of Steel for keeping a straight face to this.
From the same chapter, Subaru pretty much threatened the angry girls from Takumi's fanclub by telling them he could have stalked every single one of them if he had more time. They immediately went silent with fear.
In Chapter 86, Mimasaka reveals to have obtained a lot of information on Souma's action plan from Mitsuru's cover story on him. We then see a picture of him hiding underneath the desk of the computer Mitsuru was using, without the latter noticing. See him in the fullness of his stalker's glory◊.
Chapter 73: Hisako Arato of all people brings up this moment. She beheads a living turtle infront of the audience and opens its shell. And she doesn't even flinch at all.