“Lost Cat”
So, this isn’t exactly a word that you would find in the dictionary or a text book vocabulary list, but it did happen today. The typical formulation is actually 寻人启事 (xun2 ren2 qi3shi4; “search person announcement”), though it works to substitute other nouns for the missing. For example. Pleco uses 物 (wu4 “material stuff”) for a general posting about missing items and dog lovers should keep their eyes open for 寻狗启事 (xun2 gou3 qi3shi4). The massive BCC Chinese corpus only lists 寻人启事 as a lexical item.
“启事” itself is an interesting construction and not the usual term for notice/announcement (including: 通知 tong1zhi1, 布告 bu4gao4, and 海报 hao3bao4). The 启 morpheme typically signifies opening, starting, or enlightenment, while 事 is one of those super productive characters in Chinese, but can generally be glossed as matter/affair.