Though pinholes more typically appear in cloth shutters which are exposed to light (rangefinders, the second curtain of non-instant-return-mirror SLRs).
I've had pinholes in the past and they were usually "sharper". Anyway, *if* you can lift up the Zenit's mirror (carefully, gently!), look at both shutter curtains against the light. Even not lifting the mirror out of the way, you should be able to see something against very bright light.
My own candidate would be a more simple light leak... Are the film edges and sprocket holes also affected?