After turn 20 any alien (which are 1 or 2 or even more of them) starts to cheat-see your guys, so they meant to auto-reveal eventually (even in vanilla).
There's other thing - it is possible we might have some logical obstacle for alien hostiles to find your guys. They may intentionally, or occasionally stuck or hide somewhere. I think it's the more important thing to check. They should go for your guys after turn 20 AUTOMATICALLY.
Though we can change that number in-game, starting from turn 5 after last alien seen. And reveal not EVERY of your unit, but closest. Because there's another issue - "turn20 mass panic": when last alien(s) suddenly see (aware) of whole your team of 20 xcomers, they just panic. Of course that's some sort of vanilla "feature" and sometimes is fun, but we can avoid that by revealing only 1-2 of your units, and make everything harder and logical, i.e. as it meant to be.