Zen, I don't think the excel spreadsheet is a good idea. It is when you are the boss and are trying to monitor her. But as an employee, whose job is not affected by her, it could be intercepted as obsession, bullying and personal vendetta. The other person can make up excuses, or say all that is not true, and it doesn't matter what's in the sheet, most would be OP's word against this woman. It can certainly turn back on OP in a nasty way. It would not make her look in any way either. However, clocking her out discreetly will bring these issues to anyone in charge of time managing, and she might start to be investigated as a consequence. As a minimum, there will be an official log of her hours that she cannot deny. OP can also excuse herself by saying she has only done in on occasion when walking past the machine, as she often claims she forgets. It is not right, agree. I want her to be found out and given a good b**ing. I would like all the office to be vindicated and bring their fist down with a joyous "Yes!". BUT I think keeping a log and going to the bosses in a few months will actually look bad on OP. She has already reported it as well. Although NOT true, it would be SEEN as: you don't trust managers to do their job (which true or not, you cannot tell to your manager. You don't know if they are keeping a log already for dismissal), you cannot let go and are spending a lot of time recording someone else's times instead of doing your work (as she's not directly involved in your work, you'd have to actively enquiry, so go out of your way), you don't have new managerial skills that involve conflict resolution without picking on people's negatives or telling off (I'm sure this is what is needed in this case, pure telling off, but if OP is aiming to have any promotion in that area, it looks bad on her managerial skills), you cannot be part of a team, as you constantly look at what others do and get ( again, this not the case, but it can be seen this way), you have a personal vendetta against this worker, which can made look true if this other person makes up incidents with op of a personal nature, etc.
If you put this in a STAR style, it backfires: Situation: There was an employee that was bringing down morale because of privileges. Task: I decided upon myself that I would expose her. Action: I recorded a log and escalated it to Personnel because the boss wasn't listening. Resolution: I got her fired / My boss opened a file for bullying ... I think it looks as if OP is problematic. I know it's not the case, but she could be perceived as such. I still think there's more to it than we and OP know. The only way I'd suggest a log is if the whole department agrees to do it, everyone records it when they see it, and the most liked and trouble-free person goes with the log as representative of the office. If not, is just OP against her and it will look bad on OP as well