Shadows can mean several things to me. Yes - it's the feeling that an attack is coming on, but often it doesn't. These can last for a few minutes or simply just be sudden bursts of CH pain.
It can also be the feeling that you've been hit with a low level attack - one that isn't bad enough to treat. I get these a lot at the moment. It feels like an attack is starting but then it levels off and never really gets going. The feeling can linger for hours - sometimes even the whole day.
After an attack, I'll get constant "reminders" of the attack, as if my body is suppressing the attack but it's trying to push through again. I call these 'post-attack' shadows.
Lastly, and most bothersome to me, are the bouts of constant pains I get. I've had lots of these bouts. Some days I'll just get repeated nasal blocking on my right side accompanied with sore/burning pain that worsens with each cycle of blockage. I'll get days where I simply have a continuous burning/piercing pain behind my eye. I'll get days where I have the nagging ache in my head that I get with CH attacks but never serious enough to be a real attack. Throughout the whole cycle, I'll be in constant fear that attacks are just about to start but this has actually only ever happened once or twice. These bouts of shadows usually don't result in full-blown CH attacks. I recently spoke to another CH sufferer in the UK who recognised these symptoms in himself.
When I stopped smoking weed (heavily) aged 25 due to anxiety, my CHs started that same year. Took years to be properly diagnosed.
Stopping smoking tobacco 2.5 yrs ago has lessened the frequency/pain of the cycles. Well, until this week...
Everyone is different... with some common shared triggers/experiences. Whatever works is worth working with.