i wouldn't recommend painting on someone's wall, but if it really is just an abandoned area, and you intend to paint something artistic, well done, and interesting....then i suppose have at it.
and remember, the impression you give off could help you a lot. Walk in like you own the place, plan it out, do it nicely, and make it look like this is a thought out idea. If you look sketchy as hell, and are prepared to run off at any moment, suddenly it looks like you have much more bad intent. If you don't look too concerned, they might think you are a genuine artist trying to make some boring wall look good, and hopefully the worst they'd do is simply make you paint over it.