It souinds like what you need is a room occupancy sensor, something that tells you if there is someone inside the room rather than entering through a doorway with a security pass. RFID is unsuitable for this purpose because it's range is only a few cm, unless you use it to count people using the card to enter and leave the room.
IR sensors in a door frame will work, you use two emitters and two sensors with a space between them so you can detect which direction the interruption in the light beam is passing. The theory is you can count up the number of people entering and subtract the number of people leaving so you know if anyone is still inside. It is easy to fool though, for example what happens if you carry an object, for example a large box, with you as you enter and the system registers two people instead of one person. When you leave without the box it still thinks someone is left behind.
You could consider PIR sensors but they work on changes in radiated heat in different regions of their view so stationary people, say sitting in chairs or at desks tend to be ignored. The other alternative is a volumetric sensor, using either ultrasonics or microwaves. They are sensitive to tiny movement and maybe even air turbulence so they be a better way of checking.
Brian.