At the Battle of Badoeng Strait (Feb 1942), the Dutch DD Piet Hein mysteriously laid a smokescreen before she engaged the IJN destroyers Oshio and Asashio...This made her visible to the enemy as well as to the two US Asiatic Fleet tincans trailing her (John D. Ford & Pope). IJN records called it 'a very dark night' and show that the 2 Japanese DDs spotted Doorman's cruisers--De Ruyter & Java, which led the attack--at approximately 6km distance. After the two Dutch cruisers sped through without engaging the enemy ships, Piet Hein was then seen approaching (with her smokescreen), also at roughly 6,000 meters.
The US tincans were initially following at about 2,000 yds, but quickly closed the distance to Piet Hein as she began maneuvering to avoid the IJN ships. Unfortunately she blundered to within 1500m & a perfectly positioned Asashio launched 24" torpedoes, one of which struck the Dutch destroyer fair & square within a minute of launch & ended her (very brief) fighting career.
All of this was witnessed by the US fourpipers approaching from the rear...
At Sunda Strait the various IJN destroyers attacking Perth and Houston made their torpedo launchings (not closer than ~3500m & usually farther) followed by smokescreens as they withdrew. These were certainly observed on Houston. That night there was a near-full moon with excellent visibility--probably 12,000--14,000m or so.
So, visible at 6,000m on a dark night...and I would think perhaps twice that under conditions of better nighttime visibility.
I hope this is helpful.