But "telepathy" doesn't strike me as the only possibility. We know UFP medical technology cannot remove even crude, macroscopic instruments such as the Wire from a brain; it's implausible that Crusher and her team could have fully cleaned up Picard in the aftermath of "Best of Both Worlds", despite their claims. Picard's blood might still be full of undetectable nanoinstrumentation, and it might only occasionally gather up into larger-scale, detectable machines such as subspace hotlines.
That brings up a frightening prospect, though: that Picard could wake up one day as Locutus again.
It's unfortunate that the Borg went from "don't care about you" to "assimilate some cultures sometimes" to "always assimilates everyone" to "can't do anything except assimilating" in the span between 1988 and 2001. I thought the Borg were at their most interesting right at the cliffhanger of BOBW Part 1.