Question About Rising Psa Levels After Surgery

My father-in-law had his prostate removed a year ago. He has since had his PSA levels checked regularly. Because it is my F.I.L. I don't have all the information regarding his levels, but I know that they found out that his levels have risen. From what I understand his prostate was removed fully, but no lymph nodes.
My question is, if the prostate is gone, does this mean that prostate cells were left behind during the surgery? What else would cause PSA levels to rise?
I know they will be doing further testing, but my husband and I were just looking for some answers from other people going through this, rather than clinical answers from doctors.

this is not uncommon and is known as biochemical failure after prostatectomy. PSA comes from prostate cells so if the PSA is rising after RP then PSA tumor cells were left behind after the surgery. Metastatic disease is possible but that assumes an error in the original staging. See if you can find out the PSA doubling time (the time it takes for the PSA to double). This is the most important predictor of disease progression. He will need to be treated for this with either radiation or androgen ablation.