If it's already got macros, write another which triggers on the On workbook open event. Capture the windows user logon that way.
Fair shout. Although can't the user still "disable" any macros as it generally prompts before opening... I suppose he can always password protect a macro so it isn't accessible, and couple that with Jay's email suggestion could work, but maybe defeats the object if they can disable them upon open anyway....
Write a message on it saying ‘sod off X, haven’t you got your own work to do’? If they’re not changing it or using it in any way, would your work even be bothered? It wastes a few minutes of their time I suppose but they could just say they’re trying to get a better understanding of your role in case a job comes up again. Unless they are doing anything malicious with it (need more info on how they have caused problems in the past), I don’t think anyone will care.
Yeah theres a lot more to it, this is the equivalent of nailing Al Capone for tax evasion, it isn't the main issue
Do you know when they are looking at it? Is it during work time/on work premises or can they remote in?
In the days when we had desktop PC's in our office not laptops. Once put a fun little bit of code on a file I sent to my boss. I guess you could call it a harmless fun virus. Just a simple macro that whe he opened the file, it copied some vba to his personal macros. Then everytime he opened excel the macro triggered and started opening and closing his cd tray. Only a bit of fun as we used to wind each other up but it just shows how vulnerable your machine can be just by allowing all macros.
As MDG mentions, you could use a macro and specifically the Workbook.Open() event to track who opens it. There's a thread here that discusses how it can work. It would be dependent on the person not blocking macros at run time though https://answers.microsoft.com/en-us...my-excel/fe1bcb6d-c5d2-44b3-b1d2-d51c6a1a375b Some file storage stores the last accessed date. If you move the file to SharePoint Or Teams (which uses SharePoint to store date) then you could get the access details from the M365 audit logs. (you'd probably need to ask your admin).
define what users can open what file on the file. if its the formulas they are looking at, you can lockdown cells so that it is blank unless you are a certain user as well. If in sharepoint you can allow certain people to access certain folders/files.