Big red circle

Although German POWs often wore their military uniforms, it was important to have distinctive work clothing to make it harder to blend into the population if they escaped.

From David J. Carter:

The author holds a shirt as worn by internees and combatant POW during the Second World War in Canadian camps. ... The red circle was fabric as sewn into the space which had been cut out of the original shirt.

Jill Browne (and Robert Henderson in the comments) offer some additional details (and a fun cartoon) on The Uniform of the German POWs in Canada:

the uniforms had a big red circle on the back and a broad red stripe down the pant leg ... 11 cm wide