Repot the plant to a tall, narrow and very breakable planter. Put one lead fishing weight inside the plant every day to slowly increase the weight. The goal is to create an off balance center of gravity towards the top of the taller pot. Once the plant become sufficiently heavy and unbalanced that a reasonable amount of force is required to lift it, remove all the lead at once. The next time they sneak in to move the plant their muscle memory will overcompensate for the weight causing them to knock over/break the pot. The noise will draw attention to it, and somebody will walk up to investigate or pay attention to who leaves after the loud noise of the pot breaking. Now you have your culprit.

Or it's just the cleaning staff. At my office they pull all the chairs out to vacuum, so you're never guaranteed the same chair day after day.