If the only place you stored your mnemonic code is indeed the card, then someone must had access to it.
Just to confirm.. you generated the seed / mnemonic code yourself
on the ledger, right ? And since then you never entered it anywhere ?
If the answer to both questions is "yes", someone had access to the written mnemonic code.
Where do you keep your seed?
He answered that question already. Read the 2nd post from OP in this topic.
You might also check your system for keyloggers like azorult or hydra ( usually embedded with pirates games in torrents)
Not relevant if he is using a hardware wallet and never entered the menmonic on his PC.
And if he did, it doesn't necessarily have to be a keylogger, but basically any other kind of malware.