Trafalgar Law is the user of the Ope Ope no Mi. This fruit is called the “Ultimate Devil Fruit” because of its unique power to grant another person eternal youth in exchange for the current user’s life.
The Marines were willing to pay as much as 5 Billion Berries to buy the fruit from a pirate named Diez Barrels.
The Ope Ope no Mi devil fruit is also renowned and coveted for allowing the user to perform miraculous surgeries as well as to cure diseases without any known cures and even circumvent physical disabilities.
When Donquixote Doflamingo escaped Mary Geoise, he took with him the knowledge of the Holy Land’s national treasure, which would allow him to conquer the entire world if he were to combine it with the Ope Ope no Mi’s ability to grant him eternal life. For this reason, he sought for this Devil Fruil.
I think it’s safe to say that many people in the One Piece world know Law has the Ope Ope no Mi, especially the Marines. It’s hidden ability isn’t common knowledge, but the Marines, or higher ups at least, definitely know about it because they were willing to pay 5 Billion Berries. So why isn’t Law’s bounty much higher because of his devil fruit?
Even if the Celestial Dragons didn’t know about the hidden ability of the Ope Ope no Mi, the Gorosei most definitely know about it. Why aren’t they actively seeking out Law so he can be forced to use the ability on one of them, or if Law refuses, let the fruit reincarnate in a controlled environment.
Also, Law was a warlord for a time being, why was he allowed to roam freely and the Marines didn’t care much about him, rather than immediately seizing him. It felt like the Marines made such a big deal about the fruit in the flashback, but afterwards even if they knew who has the fruit, they don’t really give a damn.
And yes, I understand suddenly raising Law’s bounty to 5 Billion would be suspicious to the world, but I’m not asking for that. Considering his initial bounty of 200,000,000 Berries and now currently 500,000,000 Berries, it just seems like the Marines don’t really care that the “ultimate devil fruit” is just roaming around in the seas.
*Theory by Threyethian
