The A/B/NLM Note is a sheet of paper Paul finds in episode 2 in the same room as the ringing phone. It contains the letters A, B, and NLM, next to three pictures of a girl getting progressively more upset (or progressively more depressed). The girl has the same face as the one above the Flower Shack. The face leads to Care's Room.
In Petscop 9, it is confirmed that the note refers to Care, as in the Pet Info menu, NLM is referred to as Care NLM, suggesting that all pets are named respectively Care A, Care B and Care NLM. This idea is also supported as the faces on the note don't have eyebrows, with the note in Care's room saying "Your wife says, 'Care isn't growing eyebrows' "; and the face depicted in the note brings Paul to Care's room when entered into The Child Library.
It's possible that NLM means "Nobody loves me", something said by the "young person" talked about in the letter on the wall in Care's Room, and quoted in the Dr. Seuss book "Daisy-Head Mayzie", which is referenced in the Graffiti Wall.