I just had the same thing happen here. A day following a 75K maintenance and front brakes at Lexus dealer, went for a battery at NTB and they found the leak. Lexus didn't find it during a $300+ maintenance-I was told they don't look under the "cover" for routine maintenance. What a joke.
Anyway, one of the chief mechanics, whom I know and trust, said it is a fairly common thing in certain years. This was in our 05. My 03 had 127k miles on it without issue-which I just traded in last month.
My local dealer's service has fallen off substantially. It was the one of the main reasons I have purchased several Lexi (sp?).