This is very common, probably because diesel gets washed into the system from various working above. It gets trapped in the gravel in Treaury Chamber. If a group walked around and disturbed this gravel a day or two before your trip then you're likely to smell it. So, yes, it probaby arrived via the Speedwell streamway in wetter conditions but may no longer be apparent within Speedwell itself.
Also be aware that the lower part of the Treasury area has no draught, so it'll tend to dissipate most slowly in places like that.