Does anyone know what that could mean or be a sign of? What is this doctor testing for?

Have the Doctor run a blood panel that includes Bun, Creatinine and he may also want to check glucose. Just to see how your kidneys are functioning. Small amounts of protein are sometimes found in blood, albeit, not considered 'normal', the reference range of how much was detected, along with other 'imbalances' could mean anything from a mild infection-to kidneys that may be needing extra attention from more water to indepth diagnostic studies. In other words, sometimes we have a slight runny nose, it's not 'normal' but just something to keep an eye on!