Guinea pigs are different than rabbits, and neutering won't stop them from making this smell. They will do it in the presence of females, but also just in general for territoriality. They will especially scent mark if they are in a cage that's too small and are spending most of their time "claiming" territory.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry