I'm following the breadcrumbs here, might be completely off, but:

  [% IF drvAvailable %]
    [% INCLUDE makeLazyTab tabName="tabs-build-deps" uri=c.uri_for('/build' build.id 'build-deps') callback="makeTreeCollapsible" %]
  [% END %]

&&

  [% IF drvAvailable %]<li class="nav-item"><a class="nav-link" href="#tabs-build-deps" data-toggle="tab">Build Dependencies</a></li>[% END %]

in https://github.com/NixOS/hydra/blob/0d2a030661fb1a6ba3f5cb83c627a72b562ebe74/src/root/build.tt#L469-L471

While drvAvailable is defined as:

    $c->stash->{drvAvailable} = $MACHINE_LOCAL_STORE->isValidPath($build->drvpath);

So, since MACHINE_LOCAL_STORE seems to be local indeed, this tab will only exist if the derivation is available on mimas?