If you've got one cert with a subject alt name for each host, they'd see them all. If you use SNI and they have different certificates, the domains might still

If you've got one cert with a subject alt name for each host, they'd see them all. If you use SNI and they have different certificates, the domains might still be in Certificate Transparency logs. If a wildcard cert is used, that could help to conceal the exact subdomain.