That is a superb picture of a concealed J-box. When you made that hole and found the problem - on the first try - I'm sure the customer's opinion of you changed dramatically.
I frequent a DIY forum where it is common to hear someone insist they can hide the j-box 'just this once,' and not to worry, no one will ever need to get to it.
I would probably have been able to follow the circuit with my $100 toner; most certainly, the $250 toner would have found it. They're not up to finding the damaged UF, though.
