Any scan can give a false negative or a false negative, including MRI with contrast and full mpMRI. As I said above, a new member here was recently told on the strength of their mpMRI that they had a high grade cancer but the biopsy showed no cancer at all in the targeted area.
As much as you want to, it will be very wearing trying to predict all possible outcomes and then plan your next steps for each. If Dad gets a PIRADS 1, you would want to question whether it is appropriate to expose him to the risk of biopsy. If he gets a 2, then I would be more inclined to ask for a biopsy targeted to the area that could be felt in the DRE, just to be sure. However, if the uro shares the GP’s concern about that ridge / lump, he will perhaps suggest a biopsy regardless of the scan results.