It is possible, if there is confusion in a situation, or a cop is a little nervous. There have been instances of plain-looking belt buckles concealing small-caliber guns, and of buckles that look decorative, but that feature removable functioning (not just decorative) derringers. They are far outnumbered by cheap decorative belt buckles cast in the shape of guns.
It's kind of silly to broadcast that kind of thing, though. If I wanted to carry a gun, I'd get a carry permit and start lugging my Glock 20 concealed, though after a few weeks of lugging that monster, I'd probably start looking for a Walther PPK as a preferred hide-away.
For those who came into the movie late, Glock created a furor amongst the "willingly ignorant" who claimed that the guns' polycarbonate frames would make them "invisible" to airport X-rays. That's bull****, and anybody who knows anything about handguns could look at an X-ray of a Glock (or the components thereof) and make a pretty good guess about which model it is. Of course, the Glocks immediately became favorites for police use, once they were trialed. My Glock 20 is chambered for 10mm Auto, and because it is a very powerful round with high magazine capacities, it was seen as a shoo-in to replace .45 ACP, .357 mag, 9mm, etc for police work. The problem? Cops actually have to have the physical capabilities to control their sight picture when firing high-recoil rounds, and they have to train with these rounds without getting fatigued and gun-shy. Unfortunately, the trend to consider all officers "equal", regardless of their abilities in this regard led to the acceptance of sidearms with lighter loads, because some fraction of the police force was unable or unwilling to train to use the more powerful round effectively. That is sad, because with a little training, a 5'2" woman like my wife can get incredibly accurate and consistent with this handgun, and the center of gravity is so low in this design that the gun seems to climb, rather than rock on discharge, so that the handgun can be quickly brought back to target.