Unknown caller.
It's got to be them.
The Big Bang Theory Season 12, Episode 23
...as a wild adventure to UNKNOWN lands
or staying at home with a good book?
The Big Bang Theory Season 2, Episode 3
with an UNKNOWN virus!
Money Heist Season 3, Episode 4
takes a bullet to the face
from a person or persons UNKNOWN.
Breaking Bad Season 4, Episode 7
whose UNKNOWN provenance
jeopardizes
The Big Bang Theory Season 5, Episode 2