BD,
I doubt you have anything to be worried or concerned about. Your next test will probably be back at .04. I've read where many have gone thru the same thing and for what ever reason were tested again non-detect after seeing an ever so slight rise.
I am 18 months post robotic surgery and tested <0.1 for a year. I was forced to change labs and for 7 months tested 0.1. I was tested last week at the lab I had first used the first year post surgery and tested <0.1 again.
It could be the sensitivity of different machines or noise as others have noted. Any way I hope the best for you and your continued membership in the zero club.
BB