Alex rider - what happened?

Chapter 9

Yassen hung around out of sight to see what Alex is gonna do next. "Cassandra." Alex called out "What up?"
"A few books are missing."
"Oh hell no!" She exclaimed and rushed to Alex to see. "Damn! I'm going over the security tapes later! Don't you need to go any where right now?" Alex looked at his watch. "Art."
"Exactly. Now go. Shoo."
"Bye." Alex said as he walked towards the door and decided the last minute to detour towards the shelves Yassen was hiding at. "Let's go." Alex said to Yassen. "How did you know I was there?"
"I saw you in the reflection."
"What reflection?"
"Cassandra's big hair clip." Alex replied as he sat down after taking his A3 sketch book.
Alex was done before anyone else was and the teacher did say whoever that does an awesome job and finishes it fast can leave first. So Alex get to leave before everyone else does.
"They don't look surprised..."
"I always finish fast." Alex replied as he got into the car. They drove home so Alex can change and get his gym bag for Karate.