Unknown person joining my meeting

I received an email message that "so and so" had joined my meeting.... the meeting was a recurring one and the individual had joined the meeting 2 months in the future.  I signed on and started the meeting - no one was there, but  how can this happen, and further.. how can a person join a meeting two months in advance?  Does it indicate that someone is using a recurring meeting for their own purposes?  or that someone accidentally joined the meeting and somehow the date was two months away (it is a recurring meeting).  I am confused and just want to be sure that no one is able to use my scheduled meetings independent of me.