While it’s easy to be discovered on core worlds like Coruscant, on some worlds it’s much harder to be found out. They never explicitly say in canon, but I always went under the educated assumption that Palpatine was found by his Sith Master. The Sith just beat the Jedi to the punch when it came to him, in my opinion.

Sidious learned all that he could from Darth Plagueis

His master, and then killed him once all of his masters secrets were gleamed. Sidious came from Naboo, and lets face it, it’s not a core world. It has a seat on the senate, but they are pretty much on the fringes. If the books were right and he came from money, which is probably true given his easy rise in politics, then it would have been a simple matter to keep Palpatine from the Jedi. Even if his family knew of his force potential, some in the rich tried to keep their children from being taken by the Jedi.

The Jedi told their members to put past relationships aside, to try to forget their families, because those connections could be used to turn a Jedi to the dark side. This is why the rich keep their children hidden, their children are their family legacy. It would be too much of a loss to them to lose their child to the Jedi, and so it’s quite easy to see how the Sith lord Darth Plagueis might have stolen Palpatine away. The Sith are experts at manipulating others emotions.