In hot weather, slugs are attracted by any water and they operate at night and early in the morning. If you watered your peat pots and some water drained down the railing supports, they'd follow the trail of water to the plants. It's always better to water early in the morning, btw to prevent fungal infections because the plants and soil surface dry out when the sun comes up.
If you put slug bait on the soil in the peat pots, the bait - well, its a bait so it attracts the slugs to eat it but then it kills them. I'd also put a little slug bait around the bases of the deck supports so they will find the bait before they climb up to get to your plants.
Just use a very few slug bait pellets but make sure to put down a sprinkle every few days to keep the slugs away. More is not better -- sometimes the smell of the chemical drives them away if there's too much in one spot.
