The phrase "approaches unnoticed" can be replaced with a variety of synonyms to add variety and depth to your writing. Some possibilities include "creeps up unnoticed," "slips in undetected," "presents itself unseen," "sneaks up quietly," "ambles in unobserved," "arrives unannounced," "gains ground unnoticed," "encroaches silently," "stealthily advances," and "infiltrates unseen." Each of these options conjures up a slightly different image and can be used to convey a different tone or mood, so experiment with a few until you find the one that resonates best with your writing style and message.