5.3 liter engine that has a small coolant loss There is no leak that is apparent and no water in the oil Any thoughts?

I spent $2400.00 getting my GMC Serria Heades replaced at 68000 miles "Thank's GM"

'GM Service Bulletin 06-06-01-019B Crecked castech heads'

http://ww2.justanswer.com/uploads/PENTICO/2009-01-30_020951_yukon.pdf