It's only semi-cannon but in the Centauri Trilogy they where telepathic of a kind.
The Drakh could link their minds (from just two up to the whole species) togehter no matter the distance, but they can't scan other people like the other races telepath.
When a keeper is born, a Drakh takes it into a pouch on it's breast (seen when Londo got his) where it grows (within a few days). The keeper and the Drakh from in that time a link, which becomes permanent.
Through that link the Drakh can also read the thoughts of the host (or victim) of the keeper.
The books contradict themselfs in how far this can go. When the keeper first attaches to Londo it's written that the Drakh views all of Londos memories but later it's said a few times that the keeper (and thus the Drakh) has only access to the surface thoughts (like a business telepath in negociations).
Maybe at the "takeover" Londo had all his memories tumbling through his mind which enabled the Drakh to see them.
Also the Drakh use a stealth suit, like the ones that the people of the homeguard used in "war prayer" and like to appear out of the shadows and merge back into them when leaving.
And the keeper that later on takes david sheridan forms a very weak link with him, even when he's still in the urn. But the only thing he could do was appearing in his dream form time to time.
