Wow indeed! I've only ever seen the flashing on dash cam footage! I believe that is caused by an unfortunate relationship between between the frequency of the lights and the frame rate of the camera (30fps is common). Must surely mean that your eyes and brain are wired differently to the lay person to see this difference or the one you saw had an electrical fault.
Do all R171s look like this to you?
What can I say? I do notice this on a large number of cars - but I've not seen many R171 SLKs in the dark to say for certain. If it's visible on camera footage then the effect is likely widespread. If you can count the flashes per second on the camera footage then the SLK lights are flashing at a rate equal to the camera frames-per-second plus or minus the number you've counted. The same happens with some DRLs too.
And don't get me started on the internal LEDs The BMW I have is littered with them and they ALL flicker! Really, really annoying whenever you move your head you see a trail of on/off lights that dance with the movement of your head. Maybe I should drive at night with my eyes closed....
You are not alone.
I sometimes find BMWs and Vauxys have a similar issue.
Generally limited to the ones in Battenberg colours
Funnily enough, non-Battenberg BMW rear LEDs have not suffered from this since... I don't know when! My old 530D had LED rear lights and no flicker! I haven't noticed my new BMW flickering either.
Thats a first for me to here .
Id imagine got to be a fault on rear lights ?
Nope - the fault, if any, is in the cheap design of the LED drivers. Genuinely, the vast majority of people can't see this. I'm kinda cursed with the effect and believe me, it's more of a curse than a blessing!
If the right LED drivers were selected during manufacture then I wouldn't see it. But I expect that would cost slightly more.