@Helen3
Where here is where it gets tricky...
What evidence do you have that they were smoking? Well, it's pretty hard to provide evidence of a smell.
I am a smoker myself and pretty lenient about it. There are areas in my listing where guest are permitted to smoke, but it is strictly prohibited in others, like the guest bedrooms. Still, occasionally (and its happened way more often recently than ever before), so do just that.
Of course, you can tell, especially when it's something more pungent that cigarettes. Even as a smoker, I can smell it!
Proving it is quite another thing...