2013 Honda Accord V6. 110,000-ish miles. Says she noticed the pedal would go down while waiting at a stop light "a couple weeks ago." Dang! First I've heard of it. Why are women like that? 
She went to start it tonight (push button starter) and said the pedal went a good ways to the floor, much farther than usual. I had her check the reservoir, and she claims it's full.
So, MC?
Most likely. Weird thing to fail on that new a car though.
I'm going with "yes" and the car should be towed either home or two a repair shop.
In reply to KyAllroad (Jeremy) :
Agreed.
In reply to pres589 :
Yeah, fortunately, she works at a dealership and has access to some talent in the service dept.
First thought is, did she check the level by looking at the side of the reservoir or by removing the cap and eyeballing the fluid? Seems silly, but during my recent (clutch) mc failure it still looked perfectly full from the outside, but the plastic in the reservoir had actually been stained at the max line and was actually quite low when observed from the top. Maybe a slim chance on just a 5yr old car, but still worth verifying.
My money is on the master cylinder leaking into the booster. A line leak typically leaves evidence in the parking lot.
A suddenly sinking brake pedal is a classic master cylinder issue. Time for a new one. I'd find out who manufactures the original piece for Honda, then buy one of those from Rock Auto or the like. A lot of aftermarket MCs are garbage.
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