I noticed the same model being sold by several unofficial retailers

Gray market is the most likely explanation. Unsold inventory from ADs often ends up with third-party sellers who aren't bound by the brand's pricing rules. Doesn't automatically mean fake.

Could also be that the model was recently discontinued and these are the last units being cleared out. That happens a lot with skeleton pieces since they're niche.

What does the warranty card situation look like on those sites? If they're offering a stamped but undated card, that's a common gray market tell. If there's no card at all, walk away tbh.

I wouldn't assume fake just because they aren't on the AD list. Some legit retailers sell through multiple channels and just don't appear on the brand's locator. Check return policy and whether they accept credit cards.

Which brand is it? That makes a difference. Some brands have tighter distribution than others.

Also worth checking if the serial numbers match between the caseback and the paperwork. A lot of replicas get that wrong, especially on skeleton models where the movement finishing is harder to fake convincingly.

One more thing: price. If they're all within 10-15% of each other and roughly in line with what you'd expect, that's probably fine. If one is 40% below everyone else, that's the one to scrutinize.

Skeleton watches are actually easier to authenticate than solid-dial pieces in some ways. You can see the movement finishing, the jewel settings, the anglage. Post a movement macro if you end up buying one.

I've bought from an unofficial retailer before and it turned out fine, but I called the brand's customer service with the serial number before pulling the trigger. They confirmed it was a legitimate unit. Might be worth doing that.

Brand customer service wont always confirm serial numbers for gray market pieces. Some will, some won't. Depends on the company.

It's a Frederique Constant skeleton, the slimline automatic. I checked the serial number with their support and they confirmed the batch was allocated to a specific region. The retailers I'm looking at are all in different countries. That's what got my attention.

Parallel import, then. Common with FC. Not much to worry about as long as the seller is reputable.