Hello,
One more additon, I've not seen. As in the sad event in Spain, the detonations are sometimes initiated remotely. Nothing would attract more attention, than a "wired-up" (filled with some goodies including wires and some more metal parts) special mobile phone, or radio. My opinion is still to have a clearly marked (survival kit, first aid kit, etc.) box, like a simple translucent beach-box (used to store valuables during swimming), or clear lid Peli box which would allow to easily check the contents.
So this is a deal between passing more "raising eyebrows", and risking more stringent questioning after some officer popping open the battery compartment. As far something is legal, I can not see the point to hide.
Zerge