Discharge from the back passage

I have these problem. Sometimes when I release gas I have some white fluid come out of my back passage. Sometimes it is not possible to wash my back at work as there are no proper jugs to use so I just wipe it dry using tissue. I understand you must use flowing water to wash of the stool. My question is if there is no smell or physical trace of faeces are my underpants deemed still to be pure? I don't smell anything on them, I can't ascertain whether the colour of the stool gets on my underpants as they are dark colours. But there is no smell or physical stool there.