More details as below:

https://www.googleapis.com/drive/v2/files/1iNMGIAFXuhS_CO_hnEO0_EJ9PAgT-hXYqWYv0MPGUTI/comments?alt=json returned "Not Found

The file is present in drive and shows in drive.changes.list, but can't be opened in Google Drive either.

There are two problems here

a) the file is somehow corrupt ( it was a document imported into drive, so that failed, but that isn't something I care about for the purposes of this question )

b) The file shows up as existing in some API calls, but calls to read comments with the Drive SDK comments API fail.

Here are results from an API call showing how the file does indeed exist:

  "file": {
    "kind": "drive#file",
    "id": "1iNMGIAFXuhS_CO_hnEO0_EJ9PAgT-hXYqWYv0MPGUTI",
    "etag": "\"o35FABD0TC3H-Up3OL3UA9kEB2w/MTM3MTc2NzU5NzEyNA\"",

.... ....

    "iconLink": "https://ssl.gstatic.com/docs/doclist/images/icon_11_document_list.png",
    "title": "<removed>",
    "mimeType": "application/vnd.google-apps.document",
    "labels": {
     "starred": false,
     "hidden": false,
     "trashed": true,
     "restricted": false,
     "viewed": true
    },