Are you missing JUST the signs ? Check your SmallPartsRejectRadius, perhaps is set too high and is excluding smaller objects.
Also, if you are checking the scenery in slew mode, it might be normal they would disappear, because are intentionally turned off over a certain altitude from ground (you don't need them when not on ground)
And, if you use a too wide zoom settings, they might be also disappear due to LOD optimization. The scenery is designed to work at zoom levels from 0.50-0.60 and higher.
