Skulk Sensors activate when you make a noise in spectator mode.

I was flying around the Deep dark biome in spectator mode on my Nintendo Switch, and then when I passed a skulk sensor while flying (sprint flying) it noticed the "noise" I made by flying in spectator mode. The skulk sensor was naturally generated and after it picked up my spectator's flying 3 times it spawned a Warden (I didn't get the blindness effect)

To replicate this I'd suggest finding a deep dark biome or placing a skulk sensor from creative mode, switch to spectator and fly around it. I was right above the sensors when they activated, approximately 7-8 blocks above 

I apologize for not having a screenshot, but my screenshot timing is really poor.

 

The only experimental features I had active were the wild update and spectator mode experiments