I had protein spilling in my first pregnancy early on, then it stopped, then it came back after my blood pressure started rising and the pre-e set in.
This time, I had lots of protein early, but also "unmasked' chronic hypertension. When the labetalol brought my hypertension back down into the high edge of normal range, the protein stopped, so it's probably my kidneys were stressed from the previous issues and now this. (That's the theory I've gotten from the doctors so far..)
So, I understand on the mysterious protein thing. It's troubling, but without any other symptoms, you just have to keep an eye on it and keep in touch with your doctors.
1st son, induced at 37 weeks due to pre-e, born 6/18/10.
2nd son, c-section at 37 weeks due to hypertension, born 1/15/13.