Slug standing up

'Look, a slug standing up' called my 7 year old daughter. I went over by her and watched. Indeed, a Spanish Slug, Arion vulgaris, reared up as high as it could go in between some pots. It waved its body from side to side, and after some time it bent over itself turning round, touching its mucus pore with its head. I had seen this behaviour previously described in Rowson and colleagues' FSC guide 'Slugs of Britain and Ireland':

'Arionids are sometimes seen rearing up off the ground, as if sniffing the air' 

It is a curious behaviour of unclear function, are they really 'sniffing' the air? could they be sensing humidity in the air, assessing when to retreat to a refuge, or looking/smelling for food? Although slugs have very simple eyes, this rearing and moving side to side could allow them to assess where shadows are indicating plants to climb. Alternatively, they could be detecting or emitting pheromones to find or attract potential mates.