There are a variety of synonyms for the phrase "approached unnoticed." Some possible options include "snuck up on," "crept up on," "stealthily approached," "quietly advanced," "sneaked up on," "came upon unobserved," "slipped up on," "moved in unnoticed," "gained ground quietly," and "quietly made way towards." These phrases all convey a sense of someone or something approaching without being detected or noticed by others. They can be useful for describing stealthy movements or unexpected arrivals, and can add a sense of suspense or intrigue to your writing.