trust your gut and trust what you SEE. he IS using.
you did nothing wrong. he is an addict who has relapsed. and he thought he could get away with it. and he made some really bad decisions, disrespected you, got close to getting arrested, and is now LYING to your face.
no addict forgets a bag of pills. in their work bag that they use every day. i'm sorry, you may not want to deal with this again, but here it is. it is best to accept, so you can deal with this rationally and clear headed. it won't just go away. now he's cornered and going to be defensive and more secretive.