Thanks, Gurney and Will. They're important warnings.
I find the snails die inside the trap. Maybe we have a different type of poison. I'm careful to hide the trap under vegetation too. We don't have hedgehogs -- I wouldn't use snail bait if there was a chance of anything eating the baited snails. Here the only enemies of the snails are Ravens and Blue-tongue Lizards. Both hunt away from my enclosed vegetable garden.
Still ... I agree we need to be very careful with the use of any poison or indeed with all of our actions that impact on the natural world.
Cheers, Joy