@k-5ou I play Mirage pretty often, and honestly I disagree that a bloodhound scan should light up the "real" one. It makes it a complete waste of an Ult if there's a bloodhound behind you then, cause what's the point if he can just go "HEY EVERYONE, THAT GUY THERE!"
They take more bullets to disappear as people would just spray and pray to clear them out. I do agree, however, that having sound is a bit weird, but removing it entirely makes it pretty much a null point to send solo Mirage clones out to go draw fire, nobody can hear it and therefore don't care. Spotting a Mirage IS his whole ability kit; you're not really supposed to be able to. If he was so easily spot, there would be no point in playing him.
Furthermore, if you are within a 5m radius of him, you can see the discs on his body light up and give him away.
Another tip, the clones cannot climb. (Or do anything else for that matter.) So use the fact one of them is likely climbing or moving around stuff too easily.