I have had 4 episodes in the past 3 years and they all started with runny-nose, flu-like symptoms -- which is the most frequently reported symptom. The first three coincided with major holidays (Thanksgiving, Easter and Christmas, respectively), so one might think that holiday-related stress plays a role. However, holidays also bring people (with colds and other illnesses) together, so my suspicion is that it is the human contact aspect of the holidays that is our problem -- and not just the stress of the holidays -- because for some reason our immune system has become compromised.
Indeed, all 4 of my episodes were preceded by some close family member having had a cold a few days before I myself caught the cold and started an episode -- such that I now regard a cold/runny nose as the official start of a capillary leak episode. (And, indeed, a runny nose IS a form of capillary leak!).
Since my 3rd episode was cut short by the up-front administration of large doses of steroids in the ICU, I was advised to take such steroids immediately upon the start of any future runny-nose episode, in the form of 80mg/day of Prednisone pills for a maximum of 5 days, under strict medical supervision and with daily tests for blood concentration. I did as suggested when I got my 4th cold and, sure enough, the subsequent capillary leak episode was cut short and I didn't need to be hospitalized.
As concerns the MGUS protein that most patients with SCLS have, its significance is unknown. The protein is relatively common: it is present in about one in 100 people aged over 50, very few of whom end up with myeloma (cancer of the plasma cells) or lymphoma (cancer of the lymphatic system). I don't recall reading that anyone with SCLS ended up developing these cancers, so I don't worry about that outcome.