Have the Galaxy S6, 2 weeks now.  I have been monitoring this situation closely on my phone prior to reading all these comments.  While generally both lights are on continuously and see data being passed on both 4g and WiFi, I have caught both YouTube and Instagram aps actively using the 4g data, and Im now 65% into my plan... more than I use most full months.   It would seem it decides when it wants to use 4G instead of wifi (smart switch is off), but generally is not eating up data.  

I've been turning off the mobile data icon, to put a halt to it, but totally paranoid everytime I see the arrows now.  Needs be fixed please Verizon!