Some replies to the tweet said it looked like an IV had been there. That's what I thought too, but I would expect the IV needle to run straight towards the wrist. Also, in this picture it looks as if something is still under the skin between the two sores. I would not expect an IV needle to leave a visible mark where the needle had previously been.The line scar suggests something was removed.
My first thought was that it was a subcutaneous implant, like some women get for birth control. They also have subcutaneous implants as defibrillators, but these are not placed in the back of the hand. I tried to look up any implant of this type that would go in the hand, but couldn't find anything.
Maybe some kind of ID marker, one of Bill Gate's microchips?