Given the layers of dirt this symbol and the walkway were found under, I assume it was painted before the 1960s. The house was likely built between 1900-1913, with the basement being dug at the same time. But the concrete used for the floor in each room of the two-room basement is different, with the backroom where the symbol was found having more layers of dirt and what appears to be an older style of concrete. As shown in the illustrations, there is a trapdoor to the basement. At the bottom of the stairs under the dirt a painted path was found (red and green colors as indicated) which led to the back room of the basement.
The path ends at the doorway to the back room, and does not connect directly to the symbol although it shares the same colors.
I did trace the history of the home and did not find any notable family names that could have given easy clues as to the origin of the symbol. The house is one of eight roughly identical one-story brick homes on the block, all built at the same time with similar basements, but winding up with different layouts and materials today. This house was the only one with the symbol. None of the other houses use a trapdoor for internal basement access, but they do have the stairs in a similar location.
The house is located in the Austinburg neighborhood of Covington, Kentucky.
I can provide additional neighborhood history information if you think it would help. I've run this online in the past and have gotten a number of suggestions, usually focusing on a venus symbol or a half anchor. I'm curious whether anyone here can identify the symbol with a credible source.