What you need to do now is to analyze how your coins were stolen, so that it won't happen again in the future.
- have no clue how
Did you store your seed in your email, cloud storage or your computer?
- no, only on my personal file server on private network since 2013
Could your coins have been physically compromised, i.e. someone physically stole/copied your seed?
- have no idea, one wallet is safe, only default_wallet is compromised
Did you use some unsecure wallet generation method, like picking the seed words manually?
- no
Do you use a cold storage setup, or did you access your wallet from a live machine?
- live machine
Is the password on your wallet file strong?
- yes
If there's a 5 month gap between last time you accessed coins and the theft, it might mean that some malware stole your wallet file and bruteforced your password. You might want to move the coins from your other wallet to a freshly created one in isolated environment.
Your story is pretty strange, but I've seen similar stories before, and it's usually discovered that a seed or wallet file were stored insecurely and a malware incident or online hacking took place.