Commenting out "virtual" test in /usr/lib/os-probes/mounted/05efi fixed the problem. Here's an output from os-prober after change:
$ sudo os-prober /dev/mapper/isw_dahhdhgec_ASUS_OS1@/EFI/Microsoft/Boot/bootmgfw.efi:Windows Boot Manager:Windows:efi /dev/mapper/isw_dahhdhgec_ASUS_OS7:Ubuntu 15.04 (15.04):Ubuntu:linux
Commenting out "virtual" test in /usr/lib/ os-probes/ mounted/ 05efi fixed the problem. Here's an output from os-prober after change:
$ sudo os-prober isw_dahhdhgec_ ASUS_OS1@ /EFI/Microsoft/ Boot/bootmgfw. efi:Windows Boot Manager:Windows:efi isw_dahhdhgec_ ASUS_OS7: Ubuntu 15.04 (15.04) :Ubuntu: linux
/dev/mapper/
/dev/mapper/