All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
You are advised to repel such obsessive whispers and totally disregard them. When you relieve yourself and perform Istinjaa’, the presumption is that no urine is released after that. You should not consider that urine has been released unless you know it with certainty to the extent that you are able to swear to it. Rather, it is recommended for you to sprinkle water on your underclothes so that if you feel any moisture, you would consider it the result of that sprinkled water, thereby avoiding any obsessive whispers. If you happened to find any wetness on your penis or underclothes, and you did not know with certainty that it was urine, you should consider it the water with which you had performed Istinjaa’. You are advised to disregard such whispers. Yielding to such whispers leads to serious evil.
As for your suspicion that this water might have been trapped inside the penis and come out later, it is very unlikely. Apparently, this is part of the whispers you are experiencing and this is merely the remaining water that you had used in the Istinjaa', so you should repel and disregard these whispers.
For more benefit, please refer to Fatwa 280252
Allaah Knows best.