A strange behaviour was noticed over on the forum. It seems after annotation the schematic files contain a link to the cache lib. It seems strange to me that this info is suddenly required after annotation. (I would expect this to either be relevant as soon as the first symbol is added or never.)
My kicad version (same behaviour was also observed in current nightly):
Application: KiCad
Version: 5.1.4-3.fc30, release build
Libraries:
wxWidgets 3.0.4
libcurl/7.65.3 OpenSSL/1.1.1c-fips zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.20.2 (+libidn2/2.0.5) libssh/0.9.0/openssl/zlib nghttp2/1.39.2
Platform: Linux 5.2.11-200.fc30.x86_64 x86_64, 64 bit, Little endian, wxGTK
Build Info:
wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
Boost: 1.69.0
OpenCASCADE Community Edition: 6.9.1
Curl: 7.65.3
Compiler: GCC 9.1.1 with C++ ABI 1013
Build settings:
USE_WX_GRAPHICS_CONTEXT=OFF
USE_WX_OVERLAY=ON
KICAD_SCRIPTING=ON
KICAD_SCRIPTING_MODULES=ON
KICAD_SCRIPTING_PYTHON3=ON
KICAD_SCRIPTING_WXPYTHON=ON
KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
KICAD_SCRIPTING_ACTION_MENU=ON
BUILD_GITHUB_PLUGIN=ON
KICAD_USE_OCE=ON
KICAD_USE_OCC=OFF
KICAD_SPICE=ON
Digging into the code produces this comment from the header parsing routine:
// The next lines are the lib list section, and are mainly comments, like:
// LIBS:power
// the lib list is not used, but is in schematic file just in case.
// It is usually not empty, but we accept empty list.
So it looks like it was added at some point, and has just been there ever since. It appears that it doesn't actually do anything though since it is just skipped in the parser for v5/6 (in fact, everything between the opening line and EELAYER END is skipped, so it could be ignored in current versions without causing problems).