I remember being fascinated with this idea. I believe they were called the Crypteria (if I am spelling that correctly.) They were soldiers who lived secretly among the Helots after the worst slave rebellion. I think the idea was they would assassinate any Helots that could possibly be leaders of a future rebellion. If anyone can remember what this is I will be eternally grateful.
It's the krypteia (or crypteia) that you're after. But it's only Plutarch that tells us that their job was to go around hiding by day (hence (krypt-, which means "hide") and murdering (not spying on) helots at night; and Plutarch is almost totally unreliable on early/Classical Sparta.
Plato also describes the krypteia briefly in his Laws and he states that it was simply a form of military training under very harsh conditions. One thing that may be salvageable from Plutarch, though, is when he tells us in a different place Life of Kleomenes 28) that the krypteia fought in a battle in 222 BCE. That implies that they were a military unit and not a training regimen that all Spartiates underwent.