A few thoughts:
Hanlon's Razor "never attribute to malice that which is adequately explained by stupidity"
We have issues like this occasionally and all are attributed to user error. It doesn't mean that the issues aren't an issue, but id be carful with coming at something like this with a punitive approach. (I have spent several years in QC roles and honest mistakes can be misinterpreted)
AutoCAD should notify you when an xref changes, granted it wont show you who changed it.
Check with your IT group and see if they have any suggestions. Our data is backed up on a timed interval and we can go back and get our data. Short of that windows has some of that functionality built in. You can at least prove that the drawing elements were there initially. There are also file properties that are avable in the windows file explorer that can tell you a limited amount of some of the information that you are looking for. JTB has some plugins as well that show drawing specific items in the explorer window as well. If you are using hosted storage often there are ways to receive alerts when files have changed and some of these types of services have audit trails. if you are using a simple file storage and are comfortable scripting a quick google search shows a couple thing that you could to to get notified of a file change.
If it is harassment, id do two things: The first is redo your resume and start casually looking and second speak to your manager and possibly HR. If the culture is overly hostile to you it may not be worth putting in the time there. This is never an easy thing to do but sometimes its for the best.
Absolutely the most important thing is if engineering is involved and drawing content is intentionally deleted without a design basis, I would consider that a Stop Work situation and I would make it known to the engineers (especially if they are stamping). I'm all for having fun around the office, but something like what you are describing invites way too much risk.
CADnoob
