Heyo Captains!
As others have mentioned, the How-To spotting video is a great starting point for understanding how the spotting mechanic works. I would like to offer some further illustrated examples of this too!
Lets start simple; here is your Chapayev with its base concealment of 13.2 as found in the concealment panel:
In this scenario; if a ship (such as Enemy #1 is) is within of 13.2 KM of you, you will be detected:
However, if a ship is within your detection range, but has a smaller detection range than yours, you will not detect him but he will see you, such as this example destroyer who has a concealment of 6 KM,. The inverse is also true, if you are inside another ship's detection range, but he is not within yours, he will not see you :
What if a ship is outside your 13.2 KM Detection Range?
Here Enemy #2 is outside our 13.2 KM detection Range, and therefore we cannot see him with our spotting alone (ignoring his detection range at this time):
However, if that ship is outside our detection range, but within the detection range of our ally, we will be able to see him until our ally moves away/smokes/gets killed (this also goes for planes):
So what about this 28.9km "spotting range"?
In this scenario we have our ally spotting Enemy #3, but they are over 28.9 KM away from us:
In this situation, their outlines will be visible on the mini-map, but the ships will not populate in your view.
So to answer your questions:
1. The element that was providing the spotting either moved out of the spotting range or got destroyed, give the description of the situation, my guess would be that it was a plane that spotted and then got shot down.
2. The 28.9 km range is your absolute spotting range, this range does not reflect the range in which you will spot ships however, and spotting at this range will depend on other elements such as allies or aircraft.
3. It depends on a few factors:
Their proximity to you/enemy ship (are they outside this absolute spotting range)
How they are being spotted (for example, the first 6 seconds of radar use the spotted ships are only visible to the radaring ship)
however, generally speaking, yes.
I hope that helps,
Fem,