What does a bit of oil mist on rear upright mean ?

[FFMan]

Whilst the Griff was up on the ramps for its mot i always take the opportunity to have a good look underneath, and i noticed on the o/s rear upright, at the bottom where there was a little crud, this was kind of oily and stuck on, the sort of oily muck you might expect to find round a gearbox etc

The rubber boot was dry and intact so where is this oil contamination coming from ?