I'm not real enthusiastic about this; just a random idea I had;
I got this idea when trying to think of a way to enforce a solution for a talisman that doesn't necessarily save more or use a different set of skills then another solution. Currently there's no way to do this. But if the solution does involve a lemming stepping in a certain spot they wouldn't in other solutions; having a trigger similar to the secret trigger could make this work. Instead of sending you to a secret level the level continues as normal but when completed the game checks to see if the level was solved under either normal conditions or specific conditions AND if the secret trigger was reached.
Obviously this doesn't necessarily enforce one solution over another without proper backroute testing and planning on the level maker.
Or just finding a secret spot in a level could be an achievement in itself.
Secret level triggers are being removed. I'm not going to reintroduce them in a slightly modified form.
A pickup talisman seems like the most obvious way to represent something like this. And yeah, please keep it visible. Unlike other games, getting to places in lemmings tend to be slow, making exploration much more of a chore.
This actually reminds me a bit of QDots in Clones. They are basically like coins you can pick up. They don't have any effects in terms of meeting the solution requirement of the level, but when ranking your performance against other players, number of QDots collected trumps other metrics like number saved or skills used.