My brother in law recently showed me an OS map, that appears to be from the mid to late 30's, that has numerous war time sites marked on it. I've been able to work out the obvious ones, which are hatched areas indicating airfields, but there a couple which I can't. The reason I'm interested is that a couple of them are just down the road from where I live. The two symbols are a capital "A" and a circle enclosing a "+". Both are located in the same wooded area. The map appears to be a standard public OS map, so not sure if the symbols are military or something made up by a civilian or perhaps even a spy. Has anybody come across these types of symbols before?