Umm... No, let's use Jupiter instead. It's bigger, and we're all probably sick to death of jokes about Uranus.
So, Jupiter it is. From Earth it is bright enough to attract attention, and a little telescope work and a few days of motion-tracking will yield the salient facts -- it's a gas giant, it's local, and it's not coming to get us.
A light-sail would reflect more light than Jupiter does, because it's going to be reflective. After all, you can't tack against light if you're absorbing all of it. You have to be able to bounce some of it to one side in order to steer. (This implies a sail made of variably-reflective material whose albedo can be changed on the fly, but who's counting?)
Unfortunately for those scanning the heavens, the sail does not need to be that far away to get lost among the background stars, especially during a quick 'wide-angle' shot of the sky. If it's 10 or 12 light-hours out (30% further out than, say, Pluto) it might as well be invisible for all you can tell about it in a single glance.
The astute reader may wonder if this particular sail was manufactured by the same folks who make buuthandis. Without going into too much detail, we'll just say "no" and put the rampant speculation to rest. I mean, it's fun to pore over the archives and look for connections, but at some point you have to pull your head out of Uranus.