I'd agree with Oldog/Newtrick.
Judging by the thickness of the mortar bed under the sill, and the thickness of the mortar joints between the bricks, I'd say that that sill was installed by a less skilled bricklayer than the one who laid the bricks of the wall. That suggests to me that the brick sill was replaced, and I expect that "thing" is a flashing. Maybe the person doing the repair, or the homeowner at the time, thought it would be a good idea to install such a flashing so that if the sill ever leaked, the water would be diverted to the outside instead of getting into the wall where it would cause efflorescence on the wall.
I'm presuming whomever did that work was as knowledgeable as us, so I don't think it's a good idea to bury it under mortar. I would just leave it alone and let it do it's intended job. It was obviously installed to allow any water leakage to drain to the outside rather than into the wall. If it's solid and not causing any problems, why mess with it? Why not just leave it alone?
If I were you, I would just leave that sill be. However, if you do decide you want to pack some more mortar in there for good measure, I can say that I've never been very happy with the premixed brick mortar you by in home centers because it tends to loose moisture too quickly to the surrounding brick. What I do to improve the plasticity of brick mortar is to add some hydrated lime to the mortar mix. The lime helps the mortar retain water so that it remains workable for a lot longer. Keep this tip in mind if you find that your mortar dries up to much too fast.
Last edited by Nestor_Kelebay; 05-02-2010 at 08:46 PM.