Final answer:
Francesca likely missed the letter being slipped into her backpack because she was preoccupied, much like characters in literature who overlook important details due to their intense focus on other matters or emotions.
Step-by-step explanation:
Francesca did not notice the letter being put in her backpack by her friend because she was likely distracted or not paying attention to her surroundings. This is a similar scenario to characters in literature who miss details because they are preoccupied, as seen in various texts where characters are caught up in their thoughts, feelings, or actions and fail to notice what is happening around them.
For example, a character might be so engrossed in their worries that they overlook a letter, as described in the passages provided, where individuals are absorbed in their own worlds, whether it's feeling oddly happy and exuberant, furtively avoiding encounters, deep in contemplation, or even simply not being informed due to lack of communication means like a telephone.