There are many weird book markers out there. I would certainly have viewed what that individual did as harmful to the materials and, if caught, would have liked them to get some legal action uncorked on their head. I've not had people defining words, but there are many note takers, thought scribbles, and stupid underlines out there
We had one prolific fellow who wrote his name on over a hundred book spines. Willie Acre, which sounds like a nom de porn.
We have a new book section so that students and faculty can see them easily. For a few days, the area was overrun with an individual writing directly on the covers what he thought of the contents, the subject matter, and society in general. He targeted religion and philosophy, and we were concerned enough at his stance against one religious persuasion that it might lead to further book damage in the main collection or worse. In the end, after three days the offender disappeared into the night.
We had an individual who cut out the answer sections of our mathematical quiz book collection, about 50 books had to go in the trash because of them. I have a suspicion of the guilty party but I can't prove it, and it is just a hunch.