For an example, I have usb flash drive with nanowallet in it and want to keep XEMs in it. Could XEMs be stolen when an usb drive is disconnected from pc or network?
Is better to use simple wallet instead of brain wallet in this case?
Brain wallet is the most dangers type of wallet as a hacker could try to brute force the pass phrase from his PC and stuff like that already happened a lot. Many other wallets have removed the brain wallet option.
This might happen to the nano wallet as well.
I’d say simple wallet is the way to go and for extra security i’d suggest to purge the wallet every time from the Nano Wallet when you are done interacting with it. So basically you keep your wallet files on one or more USB sticks and import them whenever you need to use the nano wallet.
Please also check out How To Store Your Xem Securely
It doesn’t matter all that much where nanowallet is stored. What you need to care about are strong passwords, especially when using a brainwallet. Choose a password that is at least 40 characters long.
Also always keep a secure backup of your wallet and password or private-key. For brainwallets you only need that password to recover access to the account.
If I will use a 70 chars password generated by keepass with:
- upper cases
- lower cases
- digits
- spec symbols
- brackets
and it will be like 400 bits,
will the password be safe?
I guess that’s pretty safe as long as your PC / Laptop is not infected with a key logger or some other malware. 
Also make sure to keep your keepass file safe… if someone can hack into your keepass then the best password will be useless.
I see. I wanna keep all files offline and use them in offline pc.
There was a question about how this password looks regarding safety against brut-force attack. If it enough or I can generate fewer chars and fewer options.
I have a NEM Wallet + on my Tablet. What kind of wallet is that?
It does not seem very safe as all my funds were stolen.
You were using the android wallet.
The nanowallet is much safer than using a mobile wallet
Safe from bruteforce for sure yes.
Is anybody know if NEM has a hard wallet like Ledger or other?
trezor support is poised to be released very soon.