Are USB or flash drive considered flash memory? I'm also using it as a form of backup to my other wallets.
Flash drive? Yes. USB? You need to be more specific since some kinds of storage drive use USB port.
I'm also using it as a form of backup to my other wallets. Is it safe to use those even if you connect it to your computer which is connected online?
Not safe if you don't use any kind of encryption or have poor security practice.
It's not really on my table which can be seen by anyone. I placed it under my mousepad where no one would think something important is beneath there.
But it also took very little effort to raise a mousepad.