We collected our Avis rental car in Vienna & it had the vignette attached to the windshield.
When we crossed back into Austria from Germany by cab, we passed by some patrol cars in a minor road who looked like they were doing speed checks.
The cab driver informed us that it was a radar check to ensure that each car had a vignette and that a lot of cars without them tended to be caught on the smaller road as they did not think they would be caught that way!