Skids

Someone who normally doesn't wipe and lets the back of there underwear do the cleaning rustling is a a skid mark on there pants.

Other reasons for not wiping could be :
Busy (they might have a demanding job at Next)
Lazy

No Paper
Couldn't find a towel

They were framed

Me - man how did you leave the bathroom so quick

Skids - I don't wipe

Me - bet you've got some skid marks on your pants