[bottomcrawler]

I don't know exactly how stealthy current state-of-the-art periscopes and other masts are. But I had an idea...

I can imagine that the periscope exterior has the same temperature as the interior of the sail, or slightly warmer (some conductive heating from the sub's interior). And that temperature, in turn, reflects the water temperature of the water layers the sub has travelled through in the recent past. That could be both warmer or colder than the surface temperature at the moment the periscope is raised above the surface. If the temperature of the periscope externals is different from that of the water surface (and due to radiative cooling, it likely will be in a short time even if it started out the same), there will be a detectable contrast for thermal imaging systems.

Now, if you gave the exterior a capillary network of water pipes where surface water was pumped through continuously at all times when the scope is not in its down position, that would virtually eliminate the temperature contrast, making detection much more difficult.

Visual stealth could probably be achieved by covering the exterior with some kind of light-emitting material (fiber optics perhaps?) capable of adapting to the surrounding environment. A detector would record the current light level, color temperature and directional differences, and alter the visual signature accordingly.

Finally, radar stealth. Extensive use of RAM, non-conductive materials and careful shaping would probably be able to defeat most radars. Newer, LPI radars specialized in detecting such small objects would be harder to fool, but reducing the strength of returns still limits detection range.

Perhaps some, or all, of these methods are used today.