The AI sometimes uses colony ships for scouting, thus if they find a suitable planet it will not take time to make a colony ship to send to the planet.

In the first example it seems to be going to the star and then changes direction to the planet.

For the second in my test it actually redirected the colony ship when a scout ship found a planet around Wurmgiberg.

My experience is that the AI know some things they shouldn't know, such as the locations of things that can be mined/researched by starbases and the locations of colonized planets that they can send freighters to.

I have seen plenty of evidence that they have no idea where uncolonized planets are.  I have used Mercenary ships with a large sensor range to explore whenever I can get them.  Many times I have seen an AI colony ship that seems to be going to colonize a planet that I can see.  A few turns later I realize it is not heading directly for the planet, but it is going near it.  It does not have enough sensor range to see the planet, so it goes right past it.  Sometimes I have seen them change direction before reaching the planet because they are just scouting.

Later, a scout ship or some other ship will get close enough to see the planet.  The colony ship will then turn around and go straight for the planet.

This is why I think colony ships shouldn't be scouting.  Their purpose is to colonize planets, and there aren't any planets in the space between stars.  Colony ships should go directly to a star to see what's there.  If there isn't a planet to colonize, it should go directly do another star.  This will take some AI programming, and it may not easily be done, but it would be nice if it could be done.

I sometimes use colony ships for scouting, other times I use survey ships. Sometimes I will notice that the ai will forget to colonise the better planet in the solar system. Then I will colonise it. Then I get both planets, because of my government, and choices.

It is only cheaper and more effective if you are able to claim the planets and resources that are revealed by the scout.  Building and sending a colony/constructor takes time, and sometimes time is of the essence.

Not saying you are wrong, there are just different ways to look at it.  I will build both scout ships, and functional ships with sensor range, depending on the setup I'm given at the start.