Class 1A went to an abandoned psychiatric hospital and all the doors had blood. But one in particular was covered in blood...

They checked all the places but they didn't check the door that was covered in blood...

Tokoyami approached the door and looked closely.

Tokoyami "Should we check here sensei?" says curious

Aizawa "For what- I don't think we'll find anything"

Deku "Just a look please!" looks at Aizawa hopefully

Aizawa "Ugh. Let me see what's there." Aizawa opens the door