NO no No no... The difference is the only way you WOULDNT realize you were being hunted is if he used blood hunts active... which if i remember blood hunts the closest enemy. If you got a notification when this happened and you were above half health you would KNOW he is near you.
On top of that the only reason rengar HAS a notification at all is because he can 100-0 you without effort. If i remember when he first was released or announced he didnt actually have that notification im pretty sure. Warwick cant instakill you.
On top of that without his ult warwick can only run AT you while most junglers have an actual gapcloser that isnt their ult with the exception of like fiddlesticks who really just needs help now anyways.
TL:DR Warwick doesnt need to warn people when hes ganking as his only gapcloser without ult is running at you. And very VERY few junglers who are probably much stronger than warwick give 0 warning at all.