system
Given the following post, respond as if you're commenting on Reddit:
human
[Hercules,Disney Version] How did Hades not know Hercules was still alive?

He is in charge of the underworld, he should know every soul that comes into his domain. Why did he not instantly know that Pain and Panic lied to him? You would think he would want to gloat over a dead babies soul.

Or why did he not hear it from other gods? Yes, he mentions that he does not get out much. You would still think that Zeus's son being stolen and made mortal would be big news. Just like Zeus's son being made mortal and killed would be. One would think he would have gone to Olympus just to see the pain he caused when he thought Hercules was dead. 
He only found out 18 years later, in 18 years he never found out he was still alive.
gpt
Hades is spiteful, but he's also fairly lazy (Death is, after all, generally a reactive rather the proactive force)- indeed, assuming he's won rather then stopping to double check is a recurrent flaw in the movie. His servants told him they processed the soul, and he's got no interest in wading through the last 100,000 corpses to check. 

Ask anyone working in accounting- between sheer volume, deceitful middle management and a lazy boss, it's amazing how even extremely important things can slip under the radar.