I'll be seeing you - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Also, see you. Goodbye, as in I have to go now; I'll be seeing you, or All right, see you. These colloquial formulas do not necessarily imply a future meeting. [Late 1800s]