Early in WW2, the German Navy experimented with a small gyrocopter that was towed by a surfaced submarine. It could reach a height of a few hundred feet, greatly expanding the visible area to the observer flying the gyro. The gyrocopter was small enough that it would have been essentially invisible to ships at any distance. Never very successful, it also committed the submarine to being on the surface, and the possibility that in the event of attack, they might have to cut the gyrocopter loose and dive, leaving the observer on the open ocean alone.
Along those lines, it is theoretically possible to use a towed balloon in the age of sail... by the early 1800's, lighter than air gases had been used to carry a balloon aloft to increase the range of an observer. If the balloon were painted light blue or gray, it would have been difficult for opposing ships to spot at any distance. As far as I know, that was never actually done in a naval setting.
In WW1, some of the Zeppelins carried a small gondola that could be lowered with an observer, for use when the airship was above an overcast sky. They'd lower the observer to below the clouds, and have them report back what they saw. It wasn't used for tracking ships, but could have been. However, that would only work as stealth observation when an overcast sky was present.
Also early in WW2, the Imperial Japanese Navy had refined night time naval operations to a very high standard, both attack and observation, aided by excellent night optical gear... binoculars with very large objective lenses to amplify the available light. Using this, they were able to ambush an Australian and American fleet in the first Battle of Savo Island, sinking four cruisers with no losses. So excellent night optics could be another way to track ships without being observed... but only at night.
Edit: to show the flip side, the Japanese reliance on night optics came back on them at Guadalcanal, during the Night Action of Nov 14. IJN battleship Kirishima, along with some cruisers, were closing to bombard Henderson field, and had set fire to a couple of US destroyers, while also battering US battleship South Dakota. The flames from the burning destroyers blinded the night optics, masking the approach of USS Washington (who was using radar to track the Japanese fleet). Washington was able to close to within 8000 yards of Kirishima without being seen - point blank range for 16 inch naval rifles - and wreck it with several broadsides. Kirishima capsized and sank the next morning. This incident is also notable for being the last time that a battleship sank another battleship in combat, with no assistance from other forces.