Noticed today when scouting for the supercharger fill hole (very easy to find as it turns out). Can't say how long it's been there (could be that I simply didn't notice it before).

 

Maybe a old leak from the past.

 

I guess I would clean well (looks pretty crappy) and watch closely

 

Interesting. This person had the same residue in the same location:

https://www.ftypeforums.co.uk/viewtopic.php?t=5828

 

I had the same residual when my injectors stuck open, not sure if directly related or anything were leaking, but all pipes were replaced in the process.

Also has a consistent slow coolant loss after that, but after adding some water and tightening the cap to 3 clicks, it seems to stay at the sane level now.