While working as a government contractor at one of the US National Laboratories, many desirable (easily pilfered) metal items (tools and instruments mostly) were daubed with special red "paint" in a similar haphazard manner. This paint had special properties such that even if it was completely removed with solvents and even sandpaper or grinding, it could still be detected with a proprietary scanner. The story is that it bonded with metal at a molecular level. Stuff to be marked would be spread out on a table as described above by Zipdog, then the items were very crudely marked with the red paint.

Some Items that I used had glops of paint in very inconvenient places, and it indeed, it was very difficult to remove.

As I recall, the color of the daubed-on paint was a perfect match to what is shown in the photos.