Well, since the reason is most probably in security gap I've decided to format and reinstate full PC and HDs including passwords.

Hopefully increased security measures will help to protect assets in the future.

Thank you for comments. It realy helped to realize importance of simple security aspects.

Thank you for getting involved.

I'll reply to all questions in one message.

1. How did you generate your wallet?
    The wallet was generated by BitcoinCore software when it had been installed.
 
2. Did you put this HD in your PC sometimes to synchronize?
    Yes. It was normal HD and it was connected to PC once in a while (normally every 3 months or when transaction was required).

3. Did you save the private key / password in your pc, in a file?
    No. Neither key nor password. Unfortunately I didn't use password thinking that keeping HD away from PC and in secure place would be enough. No
    of course I will. Expensive lesson indeed:)

4. Have you tried to check your wallet address to the blockchain if your BTC is transferred to another wallet?
    Yes. Transaction is registered in blockchain. My wallet address shows empty balance. The wallet address BTCs were stolen to shows stolen amount.
    So surely transaction was real and complete. By the way the date of transaction is 30.12.2019. Hapy New Year!)

5. How did you check the BTC address? What tool did you use to check the address?
    https://sochain.com

6. What OS are you using anyway?
    Windows 7 Home Premium SP1

The think is that I also stored full blockchain on another HD which was in the home network. It was done on purpose for backup in order not to waste time if I need to synchronise wallet from the beginning. Normally it was only blockchain folder. No wallet.dat file.
However this time I realized that by mistake I transfered full wallet including wallet.dat in to the home network HD. However the network and network HD was protected by password and build in security feature. Thought it might have been stolen from there but how? It's not an easy job to hack protected network drive. And it should be done on purpose by prepared men knowing that there is something to steel. However this might have been a reason. But I'm not sure yet.

Yes. I've checked that address but it tells me nothing. Also tried to find in Google any relations with that address. No luck. However BTCs are still there and there was only one transaction on that wallet to transfer stolen BTCs. It seems it was created only for that.

I'm realy disapointed that that I don't have possibility to hack it to return stolen assets.

Thank you for commenting and tips!

Indeed security is essential.

That's why I've chosen Bitcoin Core as a cold wallet. I was sure and still sure that this is the most secure way to store BTC.

There might be a gap somewhere. HD was checked and no infections found.

I'm still looking for a reason.

Meanwhile my PC is well proected by antivirus and vpn and only 2 men on Erth know where HD is being stored.

There also might be a hack via WI FI as I'm using home network. But again it's protected.

I'll most probably ruin my brain before the real reason is found.

It's important to learn on our mistakes.

Good day to ALL.

Appreciate if somebody give advice how to return Bitcoins stolen from my Bitcoin Core Wallet.

The Wallet was kept on separate HD not connected to PC and synchronized with blockchain regularly.

After planned synchronization I found that ballance became ZERO. All BTCs were transfered to another wallet.

Can't understand how it's possible and how to recover if possible at all.

Can somebody comment on it?