We have just returned from Spain with our 3 dogs and 2 cats, all on the back seat of the car. I too was anxious regarding paperwork etc, but because the dog is not travelling back to the UK it just needs relevant paperwork stating the dog has had all need injections including rabies. We too were neither checked on the way there or on the way back. Also, don't know what part of Spain you are travelling to but we were advised to buy the collars for all the animals to protect agains sandfly etc. Another scary thing to watch out for are the pine caterpillars, they are nasty little things that kill animals and there is no prevention nor cure for them. I think they are most active though between Jan and March, but you'd need to check. Good luck.