Created attachment 397486 [details, diff] screen-ls-exit-code.patch All versions of app-misc/screen in portage tree (even 4.2.1) suffer from the bug in the URL above (from gnu-screen bug list). If you run 'screen -ls' with open screens it returns with +1 instead of 0 (which should be the right return code). The attached patch (from upstream bug) fixes it.
Fixed in >=4.3.0, then.