hmmm that is interesting, you have one single muffler that leads to that dual exit pipe, if you had a n54 twin turbo with split exhaust it could make way more sense due to a leak and etc.
How many miles are on the car? The muffler may just be old now and causing this issue somehow?
If i was you, i'd take more pictures all around and underneath the car, then take a rug and clean the tips equally, then give it a week or so to see if the same side still gets darker faster/first/only.
Never low enough.