The + symbol does not originate from the ancient mathematicians. To indicate addition, the ancients used juxtaposition with no symbol (like we sometimes do today with multiplication) or a / (like we do today with division).
The plus symbol started life as short hand for the Latin word, et, meaning “and”. The origin of the - symbol is less well known, but it shows up in ancient hieroglyphics and in early merchant documents as a symbol subtracting the tare weight from the weight of goods.
+ and - started to be used together with their current meaning in the 15th century.