It is possible. To let you know high speed cable modems all will have blinking "activity" lights, even if your computer is not on. These activities are your ISP and cable modem saying to each other "I'm Alive". Now sometimes (especially high speed) outside people will scan your cable modem. These can be live people or co-opt hacker zombie machines.
So the best way to be paranoid and keep others out of you system and relatively safe, get a router just after your able modem. If it is wireless or wired turn on the log in the router, on wireless setup wireless security using WPA or WPA2 and set a strong password.
Then on your Mac open System Preferences->Sharing and and turn on the Firewall. Once the firewall is turn on, put check marks on the services below you want to let out, like network time, iTunes sharing, etc. Then in that same pane select make sure you turn on logging and stealth mode. It is totally up to you if you want to limit UDP traffic. Lastly hit OK.
Then download the application
Little snitch. This program will monitor and report to you Outgoing traffic from you machine. This way you can monitor your system.
This way you have a good start on protecting your self at little expense. Security is not bulletproof, it is a level of protection the deters would be hacker/cracker. It will deter a hacker because the "wall" if to tiring to climb kind of thing.
