The metal is a galvanised steel lintel not a flashing.
It supports the brickwork and blockwork above the openings.
It should be fully bedded on the brickwork.
I would hope it is just a bit of pointing that has fallen out or been missed.
The gap should not be there at all and is not for ventilation. Cavities are normally filled with insulation anyway except in timber frame, which cavities are required to be ventilated but not like this!