Dave grabs it back from Dirk quickly. That is the look of someone who's obviously upset. Dave isn't some cool kid with a poker face anymore. He's just some kid who looks really upset who just remembered that his older Sis is dead.

He's already getting to feet. Kid is gonna abscond if you don't do anything about it. Like Dave needs to try and hide again because he doesn't want to deal with these emotions.

Dirk probably even saw the part where he basically deflected Terezi the entire time. The part where he admits he's not a hero. He'll never be.