ah, my mistake then. i use a wildcard dns-record but separate letsencrypt-certs for every subdomain. so to truly be stealthy i'd have to use a wildcard dns-record AND a wildcard ssl-cert.
sounds like i got myself a project for this weekend, implement a wildcard cert for my rev-proxy at home :)
EDIT: i guess the logs would still show the old certs, so my subdomains would still be exposed. huh. at least future subdomains would be hidden.
EDIT2: are there more ways for subdomains to get exposed, other than through DNS or SSL-Certs?