pinf is a tool that will Pipe an input to the terminal output If Not Found in the file given as an argument.
▶ cat "things.txt"
Zero
One
Two
Three
▶ cat otherthings.txt
One
Four
Five
▶ cat things.txt | pinf otherthings.txt
Zero
Two
Three
▶ cat otherthings.txt
One
Four
Five
Note that Pinf will not add new lines to the input file. If that's what you want to achieve, you can use anew.
▶ go get -u github.com/NkxxkN/pinf
This tool was 100% inspired from anew from TomNomNom.