Unless they can scan multiple billions of billions of star systems in full 3D (up, down etc) there will be very little chances they will know were we'll be located exactly just with this flag. Also keeping pluto on the flag as a planet is a good assurance to mess up with their google cosmo-map system, just in case...
No need for full 3d scans of star systems. Way extrasolar planets have been found is looking at changes in the position of the star as it orbits the center of mass of the system. Or looking at changes in brightness of the star if the planet and star are in the right position. If aliens can use the same methods and have more precise instruments, they can know the composition of each system. Pluto included. Doesn't help the flag shows we have only one star. That may reduce number of star systems they need to search to half or less. And if aliens see the flag somewhere, that further reduces where they have to start looking. Not like we can get very far from home for now. But that is assuming aliens understand what the flag represents. And are hostile.