How could German raiders shadow a convoy without being clearly spotted?

How could Lütjens shadow a convoy without the escort clearly spotting his ships?

He could remain far enough away to watch smoke and movement rather than individual ships, use radar when available, and exploit the convoy's predictable speed and anti-submarine zigzag pattern.

Shadowing did not require the raiders to sit within clear visual range of the merchant columns. Smoke plumes revealed the convoy's general position and maneuvers at much longer distances. The convoy maintained a known average speed while zigzagging against submarines, whereas the German surface force did not face the same need in the mid-Atlantic. Radar could supplement those visual cues when working. The raiders could follow these cues without approaching close enough for reliable identification.