What should one do who feels something exiting from the front private parts after istinja' and upon commencing wudu', but finds nothing upon inspection, and then sprinkles water on his garment and repeats wudu'? And what are the symptoms of urinary incontinence?

Your current state is one of whisperings (waswasah); you must turn away from them and not pay attention to them. Do not inspect your private parts, and do not conclude that anything has exited unless you have certain, firm conviction about it, such that you could swear an oath to it. As for urinary incontinence ( سلس البول), we have explained its ruling in Fatwa no. 97134.