Obviously, some Force users can dampen or nullify their Force signatures ( e.i. Sidious and Maul, possibly Obi-Wan). I think some Jedi who have extremely high midichlorian counts (i.e. Yoda, Vader, Luke) need to have additional Force-negators in order to hide their presences. Hence Yoda on Dagobah.

Also, it would seem that Force signature are a lot like energy radiation. If you look at the night sky while you're in the city, you're not going to see as many stars as you would outside said city (or world, if you prefer). That probably helped Sidious mask his presence.

Same with Yoda, letting his presence be shadowed by the cave.