slow leak, left rear tire...

so a couple of weeks ago i checked and inflated all the tires, as the weather changed. the left rear was at least 5lbs lower than the rest. yesterday, i noticed it visually being under inflated. when i got home i poured soapy water all over the tire looking, and found no leak. this morning i checked all with the guage and the left rear was at 10lbs, all the others were at 31 where i left them. (mine is a street only, trd sport pkg and prefer the higher pressure than the door jam 29lbs.) i soaped the tire again after inflating and found no nails or leak. until i put the water on the valve stem tip..... bubbles....

i've had the truck since april, with only about 7,000mi what gives with the valve stem not failing before? should this be a dealer/warranty repair?