The best example I can think of would be Hadrians Wall. Sources say it originally stood 16-20 feet tall and roughly 10 feet wide at it's base. In its modern condition most of the wall only seems to be about 6 feet tall. Was the stone salvaged for other buildings and projects? or did it somehow erode over time? This question goes for other buildings or structures as well.
Many buildings collapsed over time, either due to lack of maintenance, earthquakes, fires, or deliberate destruction. They were used as quarries for building material either before or after their collapse, explaining the disappearance of the materials they were once made of.
The second largest temple ever built by the Greeks was the temple of Hera on Samos. Currently, only one column remains. The rest was quarried for building materials.
It is said that an entire neighbourhood of Cairo is built entirely out of the limestone coat that was stripped off of the Great Pyramid.
In Athens, after the city was sacked by the Heruli in the 3rd century AD, a new city wall was built largely out of the blocks, column drums and other debris that the Heruli had left in their wake. Some sections of this wall remain, and you can see how they were not built out of regular blocks or bricks but segments of earlier buildings. The same goes for the city walls of many other cities; a bit of excavated Hellenistic wall on Rhodos contains column drums, and even the ring wall of the Acropolis of Athens itself contains column drums in some parts.