I enjoy exploring old bunkers from the Harbor Defense of San Francisco and have noticed a reoccurring logo (insignia?). They exclusively come up written in pen in an almost graffiti like manner. In addition the word "MARS" is ussually accompanied with it. Here are a few examples. The first one is from a barrack on Wolf Ridge and the second one is from the Radio Control Room at Fort Barry. I have also seen it in the HECP/HDCP command post in the Presidio but don't have any picture (looks almost exactly the same as the Fort Barry pic). I have looked everywhere on the internet but have come up empty-handed.
I do not have any info on the logo, but did you consider it to be a graffito? You wrote that it looks like one, and there were other graffiti in the pics.
The second picture would indicate that there was there a MARS (Military Auxiliary Radio System) Station at Ft Barry. It also appears that the insignia has other writing around it but the pic is too dark (the bottom word appears to be "support").