The phrase "pops in" refers to something sudden or unexpected happening. There are several synonyms for this phrase, including "shows up," "appears," "crop up," and "materialize." "Shows up" suggests that the person or thing arrives unannounced or unexpectedly. "Appears" is similar to "shows up" in that it suggests something coming into view unexpectedly. "Crop up" suggests something that appears suddenly and often unexpectedly, like a problem or issue. Finally, "materialize" suggests that something previously unseen or intangible suddenly becomes visible or present, like a thought or idea becoming a physical reality. Using these synonyms can help add variety and interest to your writing.