Your character has had training in how to be stealthy. They know how to move silently, stay unnoticed in a crowd or disappear into the shadows to evade pursuit. They may also be experienced with how to fool sensors and monitoring equipment in order to avoid setting off security systems. Please note that all these actions still need to be role played out. Just because you have this skill you still need to competently role play how you are getting around a security system, you don't get to just automatically bypass it. Even though your character may be familer with how to sneak around it doesn't mean they will always be sucessful at it.