What brand grip did you have? I have a D7000 with a Meike grip that does not get used that much but batteries seem to last forever. I have a Pixel brand on the D800 and Ziekos on the D90. The D800 goes through batteries pretty quickly but that is the same with or without a grip, it just pulls a lot more current than a D7000.
How did Nikon measure the current drain of the camera? If you took the grip off, did the battery last for 1000-1500 shots? There still maybe a problem with battery or camera unless they did a current measurement. I have not found any camera techs who were also electronics techs, and the job does not entail any electronics work. If a problem occurs in some module or if one is suspected, the entire assembly is replaced, or the next until the problem goes away.