The phrase "pop in on" refers to an unexpected visit or drop-in. There are numerous synonyms for this phrase, including "stop by," "pay a visit," and "swing by." Other synonyms for "pop in on" include "visit casually," "drop by," "come over," and "pop over." Each of these phrases has a slightly different connotation, but they all refer to a spontaneous visit without prior notice. Some synonyms are also more formal than others, such as "call on," "make an appearance," and "show up." Ultimately, the choice of synonym will depend on the context and level of familiarity with the person being visited.