As stated above- I'm thinking about how music is quoted from outside the work, like the Dies Irae in Danse Macabre, Sweeney Todd, and countless others. Or like how the christmassy feel in Home Alone is from Williams referencing the Nutcracker Suite, or how Braveheart references the hymn "I Vow to Thee my Country."
I feel like the term Leitmotif isn't descriptive enough, and plenty of Opera works and film works reference music that is already in the cultural zeitgeist to quickly shortcut to a mood/feeling rather than an explicit idea.
I've been calling it a zeitmotif, but that's just me being siilly- is there a more appropriate term?