Most probable answer.......
He is not a spy. They needed a target to marry to the Steele dossier and he served that purpose.
My guess, they had him targeted in. Ran an operation at him and used, without his knowledge, the FISA warrant to check info regarding him. Once Trump was a shoe in, they turned the focus onto Trump once they cleared Page of problems, hoping he would back them into something on Trump. He was an unwitting mole (dupe), opened the door to more monitoring and no one shut it because once they did, the access would be gone.
These guys are too smart for their own good by a half.