If it happens locally it should be easy to see what if anything is calling them, but...
I bet those calls come from the rust standard library. But usually if you don't call them they get optimized away by LTO, and thus they don't appear in gkrust.
I wouldn't be surprised if the instrumentation that you're adding makes the linker not remove them, or something of that sort...
I think you should be able to repro locally, if you have a release build with LTO enabled.
This check is only enabled if
-Clto is passed to rust: https://searchfox.org/mozilla-central/rev/4218cb868d8deed13e902718ba2595d85e12b86b/config/makefiles/rust.mk#258
Note that to verify that this is passing locally you may need to change this to not look at MOZ_AUTOMATION: https://searchfox.org/mozilla-central/rev/4218cb868d8deed13e902718ba2595d85e12b86b/python/mozbuild/mozbuild/action/check_binary.py#284
So that you can see
TEST-PASS | check_networking...
So tldr I think that the instrumentation the sancov flags are adding are disabling lto, effectively, or something of that sort.
So probably extending the check in
rust.mk to avoid doing this when
passes=sancov is included, or something like that, may be the best option. But this is just a guess, so verifying that locally would be useful.