it is important to let you colleagues or PM know your contract is to work 10 hours a day, so they know that you do not stay in office as late as they are, if they definitely need you for a meeting or a walk down or something, they can schedule it within your work hour, or you can stay late that day for the task but you will levae office early the next day. just sneaking out at 5 pm without letting others know ahead of time is NOT a good approach and will cause misunderstanding.
In the office where I work, many folks announce their work schedule on the Office Instant Messaging screen (you should have that in your workplace too), I think you should do the same, for example, "My work schedule: " From X:XX AM - Y:YY PM" in this way, people know your schedule.
One way or another, let others know when you will be available, when you are not instead of just disappearing from office.
Just my 2 cents, hoping it works for you.
