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The signal corps provide communications on the battlefield. The attached link explains it in detail.
Dressing rome
an air raid siren
A symbol of mourning, distress and respect
I believe you're thinking of a "cricket", a child's toy, one of the prizes you might get in a box of Cracker Jacks. These were actually only used by airborne troops, paratroopers, and only during the Normandy D-Day invasion, called Operation Overlord. The paratroopers began landing in the middle of the night, before the main force of the invasion landed from the sea at dawn. The crickets were given to the paratrooper so they could tell friend from for in the night - one click was supposed to be answered by two clicks. This was all famously depicted in the excellent movie about the invasion, "The Longest Day". Most of the time the very last thing a soldier on the battlefield wants to do is signal his position. That's a very good way to get killed.