I just completed a strike mission and after returning to the tower the postmaster had an item for me, under the heading "Lost Items", a decoherent engram.
Did this engram drop during the strike and I just didn't notice it, is that why it is at the postmaster, or is it an extra reward for doing the strike? If it is because it dropped during the strike and I didn't pick it up, will all such engrams end up at the postmaster?
Yes, this item dropped during the strike and you missed it.
Any drop higher than green will be at the postmaster when you return to the tower if you missed them in the field, not just on strikes.
I believe that they are also items that you can't pick up. I found a color shader at the lost items once. I just got done with a raid and it showed I got two shaders. I checked my shaders and I had one new one and I was at full. Then I went to the tower and found a shader at the postmaster.
