just remeber if your friend is renting and some work needs to be done on phone, electric, cable tv, water pipes, sewer, possibly gas pipes or heating fuel there is a chance someone might need to access this area. that is probably why it was there in the first place. also i would try to find out if the person living there might have gotten busted and that is why he is gone. just be safe. it sounds like an awesome area to use if you can be sure it won't be accessed by anyone and no one knows of it.