If Stonehenge was completed as is usually depicted there is now in the order of 300 tonnes of stone missing. (The visible stones are estimated to weigh 681 tonnes.)
No one can account for the "missing" stone, broken up bits that have been recovered and can be estimated to remain on site are under a tonne, and despite the oft-repeated stories of souvenir chunks being taken none have ever turned up in collections apart from scientific and museum ones.
There are no buildings nearby that have sarsen or bluestone fragments and when the nearest road was excavated no sarsen had been used to make it up.
There is an obvious attraction to the idea that the stones we see there are largely the only stones that were ever there and the monument was uncompleted.
But we may also just not be recognising where the stones went. Maybe the Sarsens were turned in to quern stones - https://www.sarsen.org/2015/02/where-missing-sarsens-went.html and the bluestones into pessoi https://www.sarsen.org/2021/02/bluestone-arse-wipe.html ?
A third theory is that Stonehenge was completed in other materials.
Could the uprights and lintels have been originally crafted in massive tree trunks which were replaced by stone as monoliths became available?
Or could another stone have been used? In 1997 Dr. Olwen Williams-Thorpe claimed there was evidence for there having been a limestone monolith at Stonehenge, but there seems to be very little evidence to back that up. Only a tiny proportion of lithic material recovered from the Stonehenge Layer in the monument and nearby is of a local sandstone or limestone. There is no evidence of such a monolith, let alone a number of them been broken up.
Which is a pity as a standing stones at Berwick St James have long been thought to be from Stonehenge but they are "Tisbury / Chilmark Stone. A single thin section was made and the rock is a biospararenite, a slightly glauconitic, slightly sandy fossiliferous shelly limestone with abraded bivalve debris." - Dr R Ixer. Not a sarsen as has been thought.
Could these be parts of the missing stones of Stonehenge? I would place the idea in the unlikely but possible category. There are, of course, much less likely theories that are commonly believed so it does not harm to float this one.