What should a person do if they notice a drop of urine leaking after istinja' and drying, and is this small amount of impurity excused?

The drop you are experiencing might be the lingering wetness from istinja (cleaning oneself after relieving oneself). The default assumption is that nothing has exited the penis when in doubt, so you should build upon this until you reach absolute certainty. If you are certain that a drop has exited, then repeat istinja and wudu (ablution), as a small amount of urine is not excused by the majority of scholars. Do not search or investigate for something to have exited, and close the door to waswas (whispers/obsessive thoughts). Do not rule that something has exited except with absolute certainty.