Well if YOU had listened to the report you would have probably found the quote from the IG to be a bit vague as to timeline, as I did. Bottom line, HUD’s books are in such bad shape, including $520 billion that cannot be accounted for, that the IG cannot even audit the agency.
The actual IG report is here: https://www.hudoig.gov/sites/default/files/documents/2017-FO-0005.pdf
If you read it, you will see that the headline claim is a false one derived that is being recirculated by people like yourself who want it to be true, without actually checking out whether the facts do support the claim - which they most certainly do not.
The real story here is that HUD has royally messed up its books. The “cannot account for $520 billion” is a gross misreading of what’s actually in the report - which is that the $520B is the absolute sum of fixes made to HUD accounting - which means if a $1m expense was removed from one sub-account and added to another, that counts for $2m in adjustments... and zero dollars to which the phrase “can not account for” would apply.
The implication that half a trillion dollars is simply missing from the department is as wrong as can be.
Let this be a lesson in the folly of relying on journalists - who generally have no expertise in the subjects they cover - to interpret data and report it correctly.