"As I was going up the stair
I met a man who wasn't there!
He wasn't there again today,
Oh how I wish he'd go away"
Paul Billson makes his living in industrial espionage. He's good at it.
He's the kind of person that you never recall talking to, you don't remember seeing. Even directly after speaking to him, you wouldn't be able to describe him or remember what he looks like. It's not magic, it's just his personality.
He's adept at disguise, and social engineering. He uses the core skills to get him into places that no one should really be able to enter. He obtains his intelligence, and he leaves as ghostly as he arrived. Security teams watching CCTV somehow view him as being unimportant and fail to really notice. He takes his time, picks his moment. He can tell when he's safe to tailgate into secure areas and when to wait for his opportunity....
Paul doesn't have super-powers, he's just unnaturally good at reading people and situations and taking advantage of weaknesses (both in people and systems).
The owners of the companies that he is targeting knows that something is going on - competitor companies are getting the commercial edge far too often.
How can they go about detecting and trapping the man who both isn't there but somehow is?