Thanks for the reply, very helpful in building up a few test scenarios on my end.
I'll just post the items here in case anyone else can add to this:
1. In the Dropbox Events tab, when are the timestamps of when your account registered the deletions? (The followup to that was: were you on the road at the time, but you answered that already!)
2. Were you syncing to the Dropbox root folder or just a single directory like /PlainText (also answered)
3. Did you delete any documents at all that day? Better yet, did you delete or move any folders in your Dropbox?
4. Were you saving the file in a shared folder?
5. If you're using PlainText on 3G/EDGE speeds, do you generally have a solid signal or is it rather spotty?
6. Do you use VPNs or have other special restrictions on your internet access for your device?
As a note about where your file was saved: It's on our buglist to fix, but here's a known issue that would explain that: When PlainText is force-quit or is terminated in the background by iOS (to free up memory for active apps), it can forget its location history if you're deep inside of folders. On the next launch it should open into the last folder as expected, but tapping the 'back' button will send you to the topmost level (in your case, Dropbox root) instead of the next folder up.