Wasn't Michael also featured in Time Magazine or Life Magazine or something like that? I can't remember whether the source for that is the movie or film, but anyways I'm leaning towards believing that Michael had to have done something rather signficant during his service and thus be noticed in the public eye. I think this because there would've have had to have been a reason for the detective who delivers the "he's a war hero" to know this. That detective is named Phillips I believe in the novel and is on the family payroll. I can't see how he would know this fact about Michael who wasn't involved in the "family business" up until this time without it having been reported in the news media. Otherwise, it may have cast suspicion on him with the pro-Barzini/Solozzo/Tattaglia McCluskey.