Nice question. This type of thing can sometimes cause confusion to a native speaker, though I think you can resolve your doubt quite easily.
The key in your sentence is "someone". This highlights an individual who may or may not exist. As such, they will only be wearing one cape, hence "a cape". Generally, I think this is a good form to follow in other situations too:
You shouldn't judge a person by their appearance. if they are sporting a moustache, it doesn't necessarily mean they're a hipster.
On the other hand, if the situation specifically deals with a group of people, we may use a plural to deal with that.
Would those people wearing capes please report to the superheroes section immediately.
Plurality is highly assumed/probable in the above example.
Finally, consider an example where we are dealing with multiple people, but referring to a singular aspect of each one. How should we deal with it? Let's see:
Can all the people wearing a cape please take it off and hand it to the nearest helper?
Singular is used to show we are referring to each individual cape. However,
Can all the people wearing capes please take them off and hand them to the nearest helper?
Plural is also possible given that we are addressing a group of people each wearing a cape.
I would say that when you want to show that the thing in question is an individual belonging (see the example below) use singular, and when it's not so important, use plural.
Men who have a receding hairline should not be considered older than those who don't. A receding hairline can be dealt with by....
It's only ever possible to have one receding hairline per person, and as such, is probably better expressed that way.