When *print-escape* is true, symbols are normally printed with package prefixes only if the current package *package* is not the symbol’s home package. It’s easy to make sure that a symbol is always printed with a package prefix, e.g. for debugging.
When the current package is the keyword package, non-keyword symbols are printed with package prefixes, and keywords are printed with their normal colon prefix. For example, in SBCL:
(in-package #:cl-user) (let ((*package* (find-package "KEYWORD"))) (prin1-to-string '(car stream :car quit))) => "(COMMON-LISP:CAR COMMON-LISP:STREAM :CAR SB-EXT:QUIT)"
For full details, see 188.8.131.52.1, Package Prefixes for Symbols.