I noticed that is a hairline crack that has developed between the tops of the tile and the underside of the coping. Also, there are several small areas where, along the tops of the tile, the grouting has flaked away. Where there is grouting missing, you can see the top of the tile. So pieces that have fallen out (all rather small really) are grout that was right up against the tile.
It has been suggested to me by my pool builder (the pool was just fully renovated two years ago so I really didn't expect this sort of problem but apparently there has been some shifting that has occurred) that the grout be chipped out and replaced with polyurethane caulking. I see the advantages to this being that the caulking remains flexible, but I'm a bit concerned and hesitant to have all the new grout chipped out. To do so could conceivably damage the tile that is directly behind the grout or even create further problems. I'm also concerned about how it will look. Will it look different than the rest of the grout around the tiles? Will the color match the grout? Basically it is only a few tiles that show the flaked grouting. The rest of the pool looks fine except for the very fine separation line has developed in some areas between the top of the tile and the bottom of the coping. My builder went around with a hammer and said the tiles all sound solid but that it does sound hollow under the coping (another thing I would not have expected with a 2-year old reno). So it appears that whatever shifting has taken place with the bond beam has already occurred anyway. I'm just having a hard time with the idea of chipping out perfectly good, new, and for the most part intact grout for the sake of putting in polyurethane caulking all the way around. Any thoughts? I'd really like to know what it will look like before I let them do it.