First, lets change thief to intruder then.

And second, in the real world, if you scan any mid to large size corporate network, and probably small ones as well, the Administrators are not going to be happy (assuming that they find out what you are doing), for a start how do you convince the Admin that you did nothing wrong?

There could aslo be IDS which you will trigger which creates Alerts and wake Admins up at 2am and make them unhappy as well.

Also, many companies will have a corporate security policy drawn up by management which will likely state that any scanning conducted against their network must be authorised by them, so you are therefore in breach of that and get in trouble that way.

So, I would tread carefully when thinking about any kind of white hat scanning, its good in theory but you could well upset people and get yourself into trouble. Especially with everyone being as paranoid as they are right now about cyber terrorism and the like..