How do you know who threw it out? I'd personally probably say something about my missing mug in front of the one who did it (if I felt certain they didn't know that I knew they did it) just to see if they would feel bad and offer to replace it. [/QUOOTE]
No. That's passive-aggressive. This needs to be nipped in the bud right now by management. No *****-footing around hoping that they'll feel bad. Hah! that will NEVER happen. This isn't 4th grade, it's a workplace. If management won't man up, then I'd confront both of them together, if possible. No finger pointing, unless the OP is positive she knows who threw away the cup. If the immediate boss won't take care of this, take it to HR. It sounds like their behavior is very disruptive in the office and it shouldn't be tolerated