So in those cases I would agree, but we're talking full pallets of bricks. Not man portable without machinery.
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Protestors in Hong Kong - very organized after months of rioting and protesting - also have piles of bricks. However, these organised piles are clearly different in nature.
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Some bricks are likely placed, such as these.
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However, large pallets like that would be easy to track via purchases. Would require specific types of vehicle and supporting machinery to unload. I highly doubt you could do this under the radar.