I'm not sure if it's the same exact scenario, but my lane departure warning seems to go off only while traveling at speed and with my turn signal on. And I think only when another vehicle is in an adjacent lane and in my blind spot.
BUT - I just remembered - that also I get what I think is a "lane departure" warning when I'm on driving around a curve and I cross over the right hand side lane markings slightly. In those instances, I do NOT have my turn signal on. But I get a warning.
So, I'm probably conflating a few different warnings systems together, but overall I have found the lane departure stuff to work. That all said I'm still new to this car plus new to "modern" cars, and on top of all of I don't drive that much. No daily commute. So I'm slow at learning all the stuff this car does...
Cheers,
--jan