@Marc Hollyoak I think you can expect this value to be returned in the next version of the API's. The REST API specs that are available from the repo indicates the same.
{
"sourceName": "source2",
"sourceUri": "https://download.microsoft.com/download/2/9/B/29B20383-302C-4517-A006-B0186F04BE28/surface-pro-4-user-guide-EN.pdf",
"sourceKind": "file",
"contentStructureKind": "unstructured",
"lastUpdatedDateTime": "2021-05-01T15:13:22Z"
}