Wandering traders drink invisibility positions when its night time. But the problem is they are still visible for zombies. I tested it several times and trader drinks the potion when there are monsters nearby. I tried putting a zombie next to the trader and he drank it. Then I killed the zombie so the trader is invisible and no monster knows where is he. After that, i spawned another zombie, and he killed the trader just after spawning, whilst trader was invisible. This means zombies can see traders even if they are invisible. Then what is the point of them drinking it?