User blog comment:HalointheSkye/Volume 3 Chapter 9 - PvP (IN WHICH A LOT OF SH*T IS HAPPENING)/@comment-25405817-20160118033856

That theory about Mercury still doesn't make sense. It would imply that they knew Ruby would be there and that she would notice Emerald among the public. There was still a huge risk that ANYONE else would have stumbled upon Mercury.

My guess, he didn't have to lay low at all. They did their move THAT day. Cinder telling him to stay hidden was just a throaway line. Unless she meant until that night.