@manutebol I'm on a somewhat similar trajectory, but a little further out. The latest was .020, also labcorp uPSA. I have noted that all value have been divisible by .004, but yours is not so maybe that is just coincidence. If that is the case though, >.006 would correspond to either .004 or .000, so the undetectability is likely a function of the effective testing range, not the actual value. So if it's less than .008 it doesn't get reported.
My urologist said he would recommend next treatment at .1, like you are suggesting. After the last uPSA, he declared me "stable." (Tonight we were joking that this is the first time anyone has ever declared me stable, which in itself should raise suspicions ;-).
When values were rising, we tested more frequently--every 3 months, but now that I have been declared "stable," it's back to every 6 months. I too had positive margin in one area, so I'm expecting it to start climbing sooner or later. Assuming it advances exponentially, 0.1 is really not that far away from .020. We'll see what comes next.