My 2 cents
In my State, possession of an ounce or less is a misdemeanor and a $500 fine. Ticket able offense. They usually only confiscate firearms and I stress usually in the case of growing and trafficking or if the firearms weren't legal, i.e. stolen, BATF violations etc.... I see a lot of people saying that your guns will be gone but possession doesn't really work like that.
1. The ways that your rooms can get caught is driving under the influence, wont have anything to do with his house or roommate unless they find a felony amount and have probable cause to thing growing and trafficking is involved.
2. A neighbor smells it and calls the police. The police do a "knock and talk" and try to determine if they can get him to admit that he has it and they can come in based on that or if they smell it. Either way it's possession and why would that be subject to your firearms if you can prove they are yours and legal.
3. Former lover or roommate snitches him out, see above.
The risk is low for you depending on your state or city laws which you can find out at norml.org
There are a lot of cannabis related forums and you could find a lot of answers. I've read tons of stories about possession busts stemming from a loud party and the like and have never heard of a single thing confiscated other than paraphernalia. Plus in 19 years of watching COPS on TV I've never seen it happen
I had to learn a ton about this and ALL the laws for my state because I had to help a very old, very sick person learn to cultivate under my states medical marijuana act. I don't use but I have the utmost compassion for those that need to.