I'm going to try and keep an eye on hidden challenges - I'm looking forward to seeing what other communities within Strava do. For example, the FebFifteen challenge was also a "secret" one that was publicised by CyclingTips and word of mouth (and Strava feeds). The Battle of June (actually it's the Batalla de Junio) challenge is associated with a bike shop in Barcelona, and the AMR series was for Another Mother Runner (thanks Google).
So while I have no intention of completing the 48 hours of cycling in 24 days challenge (Batalla de Junio), I joined up to see how far knowledge of it could spread in my little set of connections (based on assuming that I am patient zero for all my Strava networks, which may or may not be true). So far I've infected two people in Strava groups, and one of the Breeze ladies... *cue manic laughter*