Have seen several statements that Colin has been confirmed for the third season. Can’t find the source of the confirmation. However, since it is only 5 episodes, it seems likely he will return for a portion since his part could probably be filmed in a matter of days/weeks. (He appears to effortlessly, fluently fit short projects between and among the larger ones. Clever man!) 
It is questionable any intense look at DS Anderson’s past will happen, though there may be a suggestion of it. This season is meant to wrap up the story between Stella and Paul.
bbcdiscovery 
As much as I have read it on a couple of blogs, I also can’t find anything official. However, I would hope and assume that they have to complete that story line – I can’t believe they would just leave the viewers not knowing what happened to DS Anderson. I also wonder whether, with Colin included in Season 2, that there wasn’t an agreement in place for him to return should there be a Season 3.
And you’re right – there was a suggestion that there’s more to DS Anderson than we saw in Season 2, which we can hope is expanded on.
“Hope” – another word for “wishful thinking”. 🙂
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It does seem odd that they would bring a major character (he got third billing) into the last several episodes of the series and not have some kind of idea as to where they were taking the story line. Plus Colin’s remark that he felt he was more special than just a one night stand. Though I know of series that have gone off the rails over the off season hiatus, the people behind this show seem to want to keep a tight reign on the story.
If this season is meant to wrap up Stella and Paul’s story (how much further can they take it?), does that also mean the end of the series? And how much does that actually have to do with ratings and whether BBC wants them back. If ratings stay up, I would not be surprised to see another cliffhanger in the last episode. (So much to ponder! And do some wishful thinking!).
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Just guessing, aren’t we all? It seems probable that Paul’s story will finish. It also seems probable that BBC will work on continuing the same or a different series with Stella … who has become kind of an icon, and great representative of female determination.
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Of course, the just guessing is all part of the fun! How much mileage did Sherlock get from all the wonderful speculation around ‘how did he do it? ‘ And I am secretly hoping that Stella becomes the female Harry Pierce (Spooks). Everyone else may get blown up, etc., but he is immortal even to the point there is talk about bringing the series back. Actually he and Stella might make a good team! She is definitely a great example of female determination. And that you don’t have to be 18 to be sexy. (I love her clothes!).
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