check with your property management - you may be receiving cable that they purchase in bulk for the units and the cost would be included in your rent.
There really isn't any way for the cable company to realize that you're watching the service, especially if it's the analog signal (i.e. you don't use a receiver but plug the coax from the wall to the TV directly). If it is the analog signal, though, this service will only last until Feb. 2009 as all cable and satellite broadcasts will then be digital and need receivers to decrypt the signal. The cable company can, however, stop the service at will if it's not subscribed to, so it could be here today and gone tomorrow.
If this is a service included with your rent and you want more channels ask your property management how to go about it. Oftentimes you may need to call the cable company directly to order a box, but some managers have boxes that they will give you for your unit.
If it isn't a service your management provides and you want more channels, call the cable company directly and ask them about it. They can explain to you the cost of setting up the service and, if you decide to go ahead with it, send you receivers to get their full programming.
ETA: Nini, are you aware that one of the many reasons that cable and satellite prices are so high is because there are a lot of people committing what's called signal fraud (i.e. theft of services)? Part of it is to compensate for the loss in payments that a thief would otherwise have to pay, part of it is that companies are forced to constantly come up with new technologies to prevent such theft. If you're stealing cable, you're part of the problem you complain about.
Kristina, I don't mean you when I say that, just so you know. You happened to come across this problem and it's likely that it was an error on the part of the cable company. If they ask about the services, be honest - tell them that you plugged your tv to the cable outlet and saw that you had service (you don't have to tell them how long you've been watching, though, if it's been a while). Companies would rather sign you up as a customer at this point rather than prosecute (they're not out for blood like people make them seem), so you don't have anything to worry about.