How do I make a blood stain show up after being triggered?

Basically, what I want to do is set a blood stain on the floor, for example, and when the player activates some sort of trigger it will show up on the floor as it wasn’t there before.

I don’t know if this is possible with overlays or decals or whatever, but I’ve seen people do it, not only blood stains but anything ranging from decals and stuff.

Something similar to when that zombie falls down that room in the Office Complex chapter in HL1.

Thanks in advance.

env_projected_decal iirc.