Here are the chest ultrasound images from a cardiac surgery patient. They were obtained from the sternal area and represent comet tail artefacts from sternal wires. They look like B-lines but they are in the black background of sternal shadow rather than hazy background of the lung. Moreover, they are arranged in zig zag pattern unlike B-lines.

Note the chest X-ray from the same patient showing sternal wires.

In terms of mechanism, these are a form of reverberation artefacts where the echoes generated from the main beam are repeatedly reflected, in repeated trips before going back to the transducer. Remember, time = distance in ultrasound language and the machine places echoes making repeated trips one below the other (taking more time to reach the probe = placed below), finally creating a vertical line. On the other hand, lung B-lines are ring-down artefacts. Consider watching this video (20 min-27:30) if you need to review these and other important artefacts.