Hi.
It can be scary to see another side of someone you know and love.
Although it may be extremely difficult to not jump to conclusions about what this means, you really don't know... I think it would be important to hear from your father before you make assumptions about his behavior or habits.
So, I know it may be difficult but I think the best thing to do is tell him about what you found and ask him what is going on. The fact that you weren't snooping through your father's things might make it easier to bring up. You can explain that you were looking for something--and happened to stumble upon something of his that you didn't expect.
Depending on your relationship with your father, you might not feel ready to do this right away, but I think if it is really bothering you, it would be helpful (but difficult) to get it out in the open. He might not tell you the truth but it could start an open discussion.
If you really can't confront your father whatsoever, then I think it might be helpful to talk to a trusted adult. Do you have an aunt, uncle or older cousin that you are close to?
Marijuana is illegal and according to the National Institute on Drug Abuse in Washington DC, it is the most widely used illicit drug in the United States.