The difference is if its smeared on the front I would say hes been with someone.
It could be his imagination going, he could masturbating during the day on the job , or left-over come from having real sex but let's not assume this.
Once it is completely gone, give it a thorough wash.
I have asked him about it twice now.
it could be happening without him knowing it.
If it's more than just a spot or two here or there, I would possibly raise up the red flag.