The funny thing is, that the warning is correct. I made a typo -- instead of - towards the end. WARNING: /tmp/yocto-autobuilder/yocto-autobuilder/yocto-worker/res-custom-rocko-multi-v7-core-image-minimal/build/meta/recipes-extended/19_debug-builds/simple-hello-world-logging-no-sstate-no-source-mirrors_1.0.0.bb: Unable to get checksum for simple-hello-world-logging-no-sstate-no-source-mirrors SRC_URI entry simple-hello-world-logging-no-sstate--no-source-mirrors.c: file could not be found ... but why did I get it? I created a new layer with the layer tool and told it to add example, which is what I built. So there is no dependency with simple-hello-world-logging-no-sstate-no-source-mirrors_1.0.0.bb It's supposed to be just another recipe lying around somewhere which can be seen by bitbake, but which is unused.
When BitBake does the check to see if any files have changed prior to loading the cache, it doesn't know anything about which recipes are in the task graph and which aren't, thus it will calculate a checksum for every file referred to in the metadata - regardless of whether the recipe containing the reference is being used or not. So this behaviour is expected, but it is also intentionally just a warning so you can ignore it if you don't want to fix it immediately.
OK understood. Thanks!