The answer came from a surprising direction. First some insight when opening the file, it looked like it was compressed/encoded data, but there was evidence suggesting it came from a macintosh. I was thinking possible memory corruption got dumped to the file.
Then we discussed things further, who edited the file on the 7th at 1pm? What did you do? wait, you don't have office on that machine... how did you... oh. Numbers. And how did you save it? You just saved it? oh. lovely. This came as a surprise because I was just realizing the filename had NO extension on it. Not .XLS, and NOT .numbers -- I'm not really sure how that happened. But he somehow deleted the excel XLS file and created a new (no extension) pages file. I don't know if Numbers is singlehandedly responsible for both of those actions or not, but if it is, that was quite rude.
I really hate that about pages and numbers, insisting on saving in their native format by default and automatically whenever you open something other than their native. Sure, you can Export to the other format. But really, every single time you edit it? It's a shameless act from Apple, making it deliberately inconvenient to work with other formats besides their own. I'd call it shady behavior.
so, case closed. but he nuked all the formulas and a fair chunk of the formatting when he did that, so now it's his job to fix the formulas. Penance.
And that folder's been added to the incremental backup roster.
