The money is not missing, just not tracked up to expected accounting standards
Article from 2013: https://www.reuters.com/article/us-usa-pentagon-waste-specia...
Article from 2002: https://www.cbsnews.com/news/the-war-on-waste/
And you can Google [missing trillions] to find an article on the topic from any given year.
It could be a tank squadron on the books that wasn't removed when it got destroyed (or dismantled) or a whole military base decommissioned but still being depreciated.
This isn't (necessarily) "the Army spent 6.5 trillion too much in one year" nor is it "the Army lost 6.5 trillion".
A single dollar mistakenly accounted upfront can sometimes require at least twice that in corrective ledger entries.
Finally, consider a mistake of the type of over-estimating the value of a capital item by an order of magnitude. The corrective ledger entry would make it look like a huge loss was incurred in a particular year when the reality was much more benign. For the purposes of this report, that corrective entry needs to be supported by specific explanation which was missing.
Or approximately 1/3 of the of the entire us GDP, compared to the 3% listed.
Roughly speaking that's like 8.8% of the GDP of the planet. That... just doesn't make sense.