"Slip by" is an idiomatic expression that means passing unnoticed or without being noticed. There are a number of synonyms that can be used in place of "slip by" depending on the context and purpose of the sentence. Some of the synonyms for "slip by" include "escape notice," "go unnoticed," "pass quietly," "sneak past," "slip away," and "get away." These synonyms may be useful when describing a person or object that has managed to avoid detection, or when describing an event or moment that has passed without attracting attention. Using synonyms for "slip by" can add variety and depth to your writing and help you to convey your message more effectively.