Lucy actually drew on her door the moment she got the text. She trusts Gabe enough to know that at worst it's a joke in poor taste. He wouldn't tell them to do anything that could actually be harmful, and so she doesn't see how it could hurt.

She had drawn it with a marker on the inside of her door (and window) though, and after seeing the marks appear on other doors, she figures she can go for something a bit more permanent, and perhaps it should be on the outside of the door. Never mind that she's moving out, she wants Daniel to be safe too when he takes over her apartment.

Turns out engraving tools aren't actually that expensive. So she's drawn the symbol onto the outside of her door with pencil, and is now noisily engraving it into the wood with a small engraving drill.