How do they get there and how do they know?

We have two bird window feeders, one on the patio door and one on the kitchen window. These are both about six feet off the ground. What amazes me is that on many a night, especially a damp one, we find big slugs in the feeders. How do they know that food is there and how do they negotiate the route to them. To get to the patio door feeder the slugs must go up and round the door sill and then go up the glass. It is even more difficult for the kitchen feeder. The slugs must negotiate about four feet of very coarse brick wall and then get around the window sill to get onto the tray (or go the long way around the end of the sill). 

As an aside, has anyone looked at the underside of a slug as it moves along?? Quite interesting.