Congrats on the excellent post-prostatectomy PSA! Urologists consider any PSA below 0.1 ng/mL to be undetectable. The standard test measures PSA to the nearest 0.1 value and the ultra-sensitive test to the nearest 0.01.
There may be some PSA from residual prostate tissue and other sources hanging around, but if it's less than 0.1, it is considered to be zero.
Take care and be well...Tim