Yup - it happens. If you have a lot of headspace in the vessel, a drop in temp (like during lagering) will cause the gasses in the headspace to contract, creating a vacuum. I you put foil over the top, you're just going to be sucking in the air from the room. I've never used an s-shaped airlock (do they really prevent suckback?), but when lagering I always limit the amount of headspace by racking to the appropriate size vessel - no headspace, no suckback.