I have two pairs of upstairs sash windows and I want to fix some blind brackets in the top recesses of the front pair. I was assuming drilling holes wouldn't be a problem because the back pair already had brackets fixed in the top recesses. However when I tried, I got stuck because there is a metallic layer a couple of mm below the surface. So I assumed it wouldn't be possible and was about to abandon the idea, but then thought to check the recesses on the back windows, and they too have the metallic layer, and someone has managed to drill through them. Can anyone tell me what this layer is and whether it's okay or advisable to drill through it? If so does it need a special drill bit? It can't be a lintel as someone was suggesting because the metallic layer is also present on the side recesses.

Thanks