Pennsylvania courts recognize a doctrine called "constructive" possession. Under that doctrine, you will be deemed to be in possession of contraband materials so long as you are capable of exercising dominion and control over them. That's why I'm in "possession" of items that are in my house, even when I'm at work, (or in my safe deposit box, my car, my luggage, etc.)
Constructive possession doesn't ACTUALLY shift the burden of proof onto the defendant, but the simple reality is that proof that contraband was in your home is strong evidence that you knew it was there and, in light of that, there will be an INFORMAL shift of the burden on to you to explain how the contraband got there, why you didn't know about it, why you couldn't have known about it, etc.
That can be a problem for a lot of defendants. For instance: why did your friend bring marijuana in his suitcase to your home, if he didn't smoke it there? And if he did smoke marijuana while he was in your house, how could you have not known about it? Wouldn't his behavior and smell tip you off, even if he didn't smoke in your presence?
In short: it's important to be careful who you let in your home, and it's important that you know what they're bringing with them when you do. If you believe that you may be exposed to civil or criminal liability as a result of your friend's stay, contact a local criminal defense attorney to plan your options.
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Most, if not all state penal codes require a "culpable mental state" in addition to the act of possessing contraband. Generally, not only must one be in "possession" (care, custody, control, management) of contraband, the person must "know" they are possessing the contraband. Other culpable mental states may apply but knowledge is the most common. Your examples present a scary problem. Even though you did not "know" the marijuana (or child porn) was in your house or car, you sure-as-shooting could be charged with the crime. A police detective would sure wonder why you didn't know marijuana was under your couch or child porn was on your coffee table. You'd have quite a bit of explaining to do. The best practice? Know who (and what) is coming into your house and getting into your car. Protect yourself from the poor judgment of others.