Saltpetre in the Garage
Potassium and sodium nirate can be used to make black powder (gun powder), but they are not explosives by themselves. It is safe to store bags of salpetre in the garage.

If there's salpetre forming on the brickworks, I'd rather be concerned to keep the bags closed and away from that wall: sodium nitrate is rather hygroscopic and salpetre crystals on and between the bricks indicate that the wall is wet.