Despite the fact that the drugs or drug paraphernalia were not found on your person, you may still be guilty of constructive possession. That is, you were in a position to have knowledge about the item and had the ability to control or possess it.
I recently had a client that had come into my office for advice about pending drug charges. They were a 'stand up' citizen. No prior 'run ins' with the law. However, they had a friend who left some drugs and a pipe in their car. They knew he had done this, but still allowed the friend to do it anyway. It was hard to explain to the client, but they were now 'in possession' of the drugs and paraphernalia too, just as if it were really theirs.
There are two types of possession in the law. Actual or directpossession; I have the illegal item in my pocket, hand, coat, backpack...it's physically on my person, in my immediate possession and control. Constructive possession; the illegal item is in a place that I know where it is and I can have immediate access and control over the item. It's in my jacket in another room, in my room, desk or car. You have to know it's there and you have the ability to get it. It's still yours even if your buddy puts it there, as long as you knew about it. Two or more people can have constructive possession over a single item. It may mean nothing to the police that your friend tells them it's his, as long as it was in your realm of control (eg: car) and you knew it was there.