How much powder and how many primers are you talking about?? That plays a large part of it.
If you have a quantity, get an old refrigerator. Put a chain around it so the door can open approx 2-3" with a lock on the chain. If someone wants to know why you've a fridge with a chain on it in your garage, just tell them you're gonna fix it but you're scared about kids getting in it or some such.
This way the refrigerator is child proof, comparatively fire proof, but if the house was consumed by fire, and the heat got high enough to ignite the powder, the pressure would open the door so it would merely burn but the chain would restrain it from opening all the way. The primers popping would be confined inside the refrigerator.
If you have say two 8# kegs and 4 or 5 1# cans and a couple of thousand primers that you don't want to keep in bulk at your bench, put them in a plain brown box, seal it and write "RLDG STF" on it and put it at the back of a low traffic closet. If you wife wants to know what it is just say offhandedly, "oh, just some reloading junk, I'm not using right now."
I agree that advertising that you have gasp shudder a stock of flamables at your home isn't a good idea.