Little black triangles

[Espada III]

Inside the cabin of an A320, irrespective of the airline, and stuck on the wall above the windows of maybe no more than two rows is a little black triangle. What does it signify?

[El Bunto]

Indicates windows from which to observe wing control surface status and icing.

[SpringHeeledJack]

I thought this was going to be about stealth drones for a second


SHJ

[El Bunto]

The triangles date-back to at least the A300-600, I remember seeing them in Monarch's cabins.