You can't know because you've never seen their car in action. If the engine has a ton of crap in it, like sludge & such, then the compression goes up & a higher octane gas is required. Or they had a bunch of crap in their tank, or they're burning oil, etc. Etc. Which is quite literally common knowledge. They might have mistakenly watched the startup emissions & took that as an indicator of engine health & prematurely changed the octane level to compensate. All of our XG's do that at start up, until the cats get hot. I've dabbled with octane levels and also found the tiniest increase in performance at high speeds over 87. Gas milage was about 1mpg better at first. Mine is the 2002 XG350L btw, with only 74,200miles on it. Mine only complaints about the car is that there are no bolt on body kits still being made here or in Korea (there were, but they're all sold out) , my Trans fluid pump is whining it's head off & needs replaced, and that they changed the location of the trans filter from being on top of the trans to being behind the pump which is completely unacceptable & requires at least $1k of labor (often) to change a $20 filter.only change the octane rating you put in your gas tank if you get a pinging noise.
other than that ONLY use what is stated unless you just want to waste money
grey smoke coming out of your muffler from putting in 87 octane?
i don't think so