Yellow box after minimizing/switching tabs

[jnewman67]

in IE 7 & 9, i've noticed that if you switch away from the game (minimize it or switch tabs) and come back to it, one of the cannons (or other guns i guess) will have a yellow box on it. it's not the last one placed, nor the last one updated. not sure how it picks one, but once it starts, it's always the same one.

photo attached.

it does nothing negative that i can tell - just weird that it appears when it's not a visual that i can make appear during the game.