Is the prayer valid if there was a very slight dampness that was only noticed after close inspection, having been felt by the worshipper on the way to the mosque, but he paid it no attention until he finished praying?

If you are doubtful about the presence of a small amount of urine on your garment, and whether it occurred before or after the prayer, then your prayer is valid. The emission of urine is considered to have occurred after the prayer, because the general principle for any incident is to assign it to the closest possible time. However, if there is an indication that the urine was discharged before or during the prayer, then your prayer is invalid and must be repeated after performing ablution. This is the view of the majority of scholars.