That's a fantastic question, I would love to know that as well. My worrying suspicion is that yes, you are. I've never heard of prosecutors needing to prove knowledge for drug posession charges.

Even if storers aren't liable, there's a very high chance you would be arrested and potentially charged, and the charges would later be dropped. Assuming we're talking about a big illegal thing here like child pornography or large amounts of drugs, that is. I don't see any way the police seize something like that and don't arrest anyone at the scene.