I too was having the exact same issue: driver's side rear blind spot monitor was issuing false triggers. Usually only when first driving the car and/or at slower speeds. Sometimes the alert would persist for a minute or two.
Last week, my dealer replaced both rear BSM modules. I haven't had a single false alert since those modules were replaced (had my car back for 5 days now).
I should note, that while they had the car, they performed the
recall for the software update related to the AVAS system. I can't say for certain if the module replacement or the software update was the ultimate solution, but there's my experience.