@oberon: yes, one Manjaro and one Windows installation.
These are the grub menu entries
Extended options of Manjaro Linux:
Manjaro Linux (Kernel: 4.10rc3-1-MANJARO x64)
Manjaro Linux (Kernel: 4.10rc3-1-MANJARO x64 - fallback initramfs)
Manjaro Linux (Kernel: 4.9.4-1-MANJARO x64)
Manjaro Linux (Kernel: 4.9.4-1-MANJARO x64 - fallback initramfs)
Manjaro Linux (Kernel: 4.4.43-1-MANJARO x64)
Manjaro Linux (Kernel: 4.4.43-1-MANJARO x64 - fallback initramfs)
Windows 10 (loader) (on /dev/sda1)
Windows 10 (loader) (on /dev/sdb1)
Last both entries for Windows 10 are the ones, which are not generated after update of actual running kernel (and only after a reboot and repeating grub-update). (Windows 10 of /dev/sdb1 is the Windows system generated partition and not a running Windows system).
And I can see, that the pacman routine proves the partitions (10 seconds with HD activity), but doesn't recognize the other operating systems (Windows) at this moment. And it's only in the case of updating the actual running kernel and the mkinitcpio had generated the images for this kernel. The Manjaro WIKI says on the informations of restoring the Grub Bootsector, that os-prober is necessary to detect if there are any other operating systems present. Seems, that os-prober is loaded or called in this case (or failed?)