Aside: On my steam version this achievement's title is actually "Cleaning tomcat".
I counted a total of
interactions with mirrors during my gameplays. It counts if you receive +3 Egopoints upon triggering and the mirror in Poirot's apartment is useable multiple times. It's best to interact with them as soon as entering the respective location because they are guaranteed to not progress the story.
List of eligible mirrors (ordered by occurrences)
*** Spoiler - click to reveal ***## Case 1: Andouver
location: MRS. ASCHER'S BEDROOM
💠 [#1/11] mirror (next to the kitchen)
## Interlude: Whitehaven, Part I
location: POIROT'S APPARTMENT
💠 [#2/11] mirror
## Case 2: Bexhill
location: INSIDE 'GINGER CAT RESTAURANT'
💠 [#3/11] mirror (left wall)
location: THE BARNARDS' LIVING ROOM
💠 [#4/11] mirror (over fireplace)
location: BETTY'S ROOM
💠 [#5/11] mirror (right wall)
## Interlude: Whitehaven, Part II
location: POIROT'S APPARTMENT
💠 [#6/11] mirror
## Case 3: Churston Revisited
location: FOYER
💠 [#7/11] mirror (above phone)
## Interlude: Whitehaven, Part IV
location: POIROT'S APARTMENT
💠 [#8/11] mirror (before talking to Donald Fraser)
💠 [#9/11] mirror (after using the phone)
## Case Closure: London
location: CUST'S ROOM
💠 [#10/11] mirror (over sink)
location: POIROT'S APARTMENT
💠 [#11/11] mirror
→ 🏆 Cleaning Tomcat