[Mänwe]

Aganzir is correct with his statement,

"'It is long since any of my own folk journeyed hither back to the land whence we wandered in ages long ago,' said Legolas, 'but we hear that Lórien is not yet deserted, for there is a secret power here that holds evil from the land. Nevertheless its folk are seldom seen, and maybe they dwell now deep in the woods and far from the northern border.'" - (Chapter 6, Lothlórien)

(However this is not really related to the thread, as the brothers originated from Rivendell.)

I surmise that it would be through Lothlórien that the brothers passed, in order to give word to Celeborn and Galadriel of the Fellowship's coming, it was their task at first to scout ahead and prepare the way for the Fellowship.

Perhaps it was this errand that they kept secret.

"2951- Sauron sends three of the Nazgûl to reoccupy Dol Guldur." - (Appendix B, The Tale of Years)

The reoccupation may have required them to journey into the lands known as the Brownlands, south of Dul Guldur. With three Nazgul in its vicinity at times, I think that sufficiently important to send ahead scouts to watch for any movement. As Aganzir has said, "his spies could be everywhere".

The discovery of the Fellowship would have been of grave misfortune.