I just spent 3 hours looking for that texture and I found it, but at the same time, did not. I found multiple copies of that texture, but changing them had no effect on the sign in game. If you search for "gov" in openiv you will find a bunch of ydr files that have "govdec" in their name. In the folder (rpf) that they are residing in are the main ytd and ydr files for that NOoSE building. After hours of toying around with it, I managed to change the smaller blue sign (located to the left of where you took your screenshot), and the sign on the blue brick wall (located behind your screenshot), but that is it. I've checked every file in that folder, and any that contained the texture for the sign you screenshotted, I changed. I even edited embedded images in the ydr files it had no effect. I'd love to know if you've made any progress finding those files since your original post.
-Jack