It was Gus' first appearance. I didn't think he would have a large role this season. At least not early on. He was an extremely cautious person. I imagine he saw Jimmy acting suspicious on his video feed in his office and Gus coming through sweeping was an alert to his guy to cancel the drop.
I figured we'd see a little cat and mouse game as Mike and Gus feel each other out. When we met Mike in breaking bad he was Gus' top guy. He enforced. He investigated. He made pick ups. He distributed money to people. Right now he doesn't even know Gus exists.
There's a lot of story to tell about how Mike got involved with Gus. A mutual dislike of Hector Salamanca is a great start.