I think there was a scene in "Sign"
There's one scene in POTN (Painter of the Night)
oooh I've been looking for some too but here's some I know
http://www.mangago.me/read-manga/encirclement_love/nbt/mpn_chapter-2187219/pg-1/
^ not mirror but a glass wall where other people can see
http://www.mangago.me/read-manga/yahwacheop/uu/iur_chapter-31/pg-1/
http://www.mangago.me/read-manga/window_beyond_window/uu/window_beyond_window-chapter-26/6/
http://www.mangago.me/read-manga/yuri_on_ice_dj_no_love_no_life/
http://www.mangago.me/read-manga/fudanshi_kun_no_honey_days/mh/c001/7/
bless your soul
I just finished reading Walking on Water in one go and it made me realize that I low key have a think for when the seme makes the uke watch them have sex in a mirror so if any of yall wanna join me in hell and provide me some links I will be forever grateful.