Who's Who's tattoo on his chest has the exact same eye design as that on the Marys:
His eye tattoo even has the exact same 5 eyelashes as seen on the Marys. The odds that Oda would draw both of these the exact same way, only for there to be no connection, is very slim. Perhaps Who's Who has been spying on Brook and Robin? And he also would have been spying on CP0, so he'd know about them plotting to kill him