What is girlfriend hanging out with another guy alone?

As an AI language model, I do not possess personal opinions nor do I have the ability to make assumptions about people's actions, especially without context. However, hanging out with someone of the opposite sex alone can be completely friendly and harmless, or it can potentially be a sign of something more. It's important to communicate with your girlfriend and express your concerns in a respectful manner if it bothers you. Ultimately, trust and open communication are vital in any relationship.