We should note that "where is there a line missing from this poem: ____?" contains sufficient information to answer correctly without needing a copy of the original to compare to.
Here are two verses of a poem (song) in Mandarin Chinese:
yi quan ting ni de
er gei ni hao de
shu dao san yong yuan ai ni yi ge
si bu hui fan cuo
wu bu hui luo suo
shuo ni xiang shuo de
zuo ni xiang zuo de
bie pa shi bai yin wei ni you wo
pei ni kan ri luo
pei ni yi qi chang wan wo men ai de ge
I removed two lines. Where did that happen?
Would your answer be different if I told you that I might or might not have removed some lines?