Favorite Son - S1-E6
Revealing mistake: When Lucifer is going through the forensics bag next to the dead body at the start, the body of the victim can be seen breathing. (00:07:30)
Favorite Son - S1-E6
Revealing mistake: When Lucifer takes off in Chloe's car during the stakeout, tyre marks are visible in the gravel from previous takes. (00:32:10)
Answer: It's Lucifer's blood from when his wings got hit by the bullets. As Lucifer spreads his wings, they're white, and before he wraps them around Chloe, you see a bullet hit his wing above his right shoulder, and the spot turns red. After that, he continues to get shot in the wings before fleeing.
Bishop73
Agreed. His wings take damage and "bleed" similar to how he bleeds when Decker is around.
Ssiscool ★
If they were normal bird's wings, the lower part where there are just feathers should neither bleed like that nor be able to stop bullets, but they are angel's wings, so they behave differently.