If I remember Boromir turned up apparently 'out of the blue' at the right time at Rivendell. Probably an argument for Eru's noodly appendage playing a part.
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However I've wondered if Elrond generally sent some sort of 'confuslement field' type vibe (magic whatever) to keep Rivendell hidden from undesirables, but could switch it to an 'attraction field' when there was a visitor who was welcome. Speculation I know but Saruman ceratinly sent some 'force' that hindered the three hunters and someone commented on how difficult Rivendell was to find (perhaps Gandalf in The Hobbit?)