OnGUI - Prefab instances,

Hi, I am using onGUI to draw a target box around a clicked flying target. My problem is that there are multiple prefab instances using the same code, so when I click on one target it does get a target box, other target boxes also start appearing around 'empty' objects which I presume are the other prefabs also drawing the target boxes from their onGUI code. Is there any way to prevent this? I had assumed a single camera being active on one instance of the prefab would be adequate, but apparently not? Thanks David,