When someone says "they are on to me," it usually means that they think someone has discovered something they were trying to keep hidden. Some synonyms for this phrase include "caught on," "figured out," "discovered," "uncovered," and "unearthed." Other variations of this phrase could include "they know my secret," "they've caught wind of," or "they're hip to my game." Whatever the case may be, if someone is "on to you," it's important to assess the situation and determine the best course of action to maintain your privacy or protect yourself from any potential fallout.