This could also be phrased as, "Why can Luke feel the place strong with the dark side and not one Jedi suspected Palpatine?" But I don't want to restrict it to just those two situations.
In The Phantom Menace, Yoda says, "Hard to see, the dark side is." While they don't think the Sith could have returned without their sensing it, Jedi regularly interact with Palpatine and never suspect him of being a Sith Dark Lord. So there's good reason to believe the dark side is hard to sense, even for a Jedi Grand Master, like Yoda.
On the Death Star, Obi-Wan knows he his facing his destiny, but there's no indication he expects it to be Vader, yet Vader knows Obi-Wan is on the Death Star.
But on Dagobah, Luke senses the place that's infused with the dark side of the force and he's a rather inexperienced Jedi in training.
Is there something special about that spot on Dagobah? (I've already read the question and answer here about the dark place on Dagobah, but there's still no clear answer what caused it or why Luke could sense the dark side there, even with so little training.
Is it just that one spot that's easy to sense? Or are there conditions that make the dark side easy to sense?