‘Beware of Dog,’ Even if There’s No Dog
There’s nothing wrong with putting up a sign that reads ‘beware of dog,’ even if you do not have a dog. Burglars are not fond of dogs, for obvious reasons, so this could make them think twice before entering your home. You can get an even better sign that reads ‘guard dog on duty,’ which would also do the job.
If you do have a dog, it is even better, especially one that is protective of its territory (which is most dogs, to some extent). Just as an aside, please do not leave your pets home alone when you go on vacation! They’re not there to guard your house, after all.