Up/Down arrows in notification tray after unlocking Just recently, I have noticed an icon that has an arrow pointing up and an arrow pointing down appear for a split second after I unlock my phone. It goes away so fast I can't get a screenshot. What is this icon? What does it mean?

My phone is a Nexus 5x with Marshmallow 6.0.1.
Turns out it was the app Signal Spy. It shows an up/down arrow while it is inspecting what your signal looks like.