"Harmlessly" pushing an unlocked door open, and walking in and writing on
a whiteboard "Gangstaboy was here" is just as much of a crime as
unauthorizedly logging into a machine, no matter how "wide open" it is,
even if "harm" is not done.
I will freely admit I have seen and peeked into "wide open" websites in the
past, either through server misconfiguration or faulty scripts. But I have
never gone much further than maybe figuring out what was wrong and sending
an email to the owner. Most people are pretty receptive if you just send
them a kindly worded email.