Here’s a place to collect gifs of the Si – Concerned Scowling signal, which is described thusly: “The scowl contracts towards the center, concentrating the muscle tension only between the eyes."
The gifs don’t need to show just this one signal in isolation, necessarily – as long as the relevant signal is clear.
Here’s Andrew Bird, who’s in the database as NeTi l—, doing a concerned scowl.