The fetcher doesn't log it's stdout or stderr to the task's output files as it does on Linux. This makes it extraordinarily difficult to diagnose fetcher failures (bad URI, or permissions problems, or whatever).
It does not appear to be a glog issue. I added output to the fetcher via cout and cerr, and that output didn't show up in the log files either. So it appears to be a logging capture issue.
Note that the container launcher, launched from src/slave/containerizer/mesos/launcher.cpp, does appear to log properly. However, when launching the fetcher itself from src/slave/containerizer/fetcher.cpp (FetcherProcess::run), logging does not happen properly.