I thought this was the best game of a few years ago and wondered if a follow up had ever been announced, or if not a direct follow up, a game by the same team?

I'm not aware of a followup game, but maybe someone else will know for sure.

It looks like the developers of Lost Horizon have since released Secret Files 3.

Secret Files 3 is made by Animation Arts, the same developer that made Lost Horizon (according to Mobygames). The Animation Arts website also lists an episodic game called Columbus 4: Der Fluch von Oak Island (The Curse of Oak Island).
Here's the ***Google translated version of the webpage for Columbus 4*** which was originally in German.

Thanks, didn't know they made SF3 (already played it). There seems to be quite a few of these episodic games now.