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Though in a more serious answer, that involves more invasive snooping, I.e. Gren bugging Sapphire's Scroll at some point to keep track of her. He already does it to Shiro's Scroll (Though to be fair it was Gren's originally) and Abel's, and would be doing it to everyone elses too if he can stick near their Scrolls for about an hour. All Gren does is piggyback off your Scroll's GPS, so as long as you update it since the last time he snuck his bug in, he could track you. He could also do it in other ways such as sending a text, which is much easier to do with his teammates since they text each other more often then they'd next others.

As for why Gren would want to know about Sapphire's honorary uncle, it was mostly coincidence. At some point Gren just wanted to know why she went to hang out with Oswald, then found the karaoke place. Gren does a bit of research about the owner and, unless he too has lived a complete life of nothingness, Gren would have found that the owner is a friend of the Rode family.