Maybe someone can help me understand how this could have occurred.
Indeed, if you had coins for months and now they were transferred, it's either your computer infected, either you've updated your Electrum from a wrong/malicious place.
how could this even happen? Is this a real Trojan attack where they scan for open wallets?
I will come with some guesses.
They can scan for what processes are open. If Electrum comes up, the trojan can send "home" various info, including your keystrokes (i.e. your seed if you enter it, or your wallet password). If you have the wallet file in a standard location, that can also be sent.
Of course, if the trojan is the electrum itself (hence the question where it was downloaded from, was it verified), then it's even easier to steal.
I know the money is gone, and I’ll have to live with that.
Make sure you never use that wallet/that seed again. I recommend you either get a
hardware wallet, either learn
cold storage (cheapest is an USB stick with Tails OS you can boot from, with no internet ever, but it's not so easy/straightforward). Also make sure you
generate your wallet offline and the new seed never goes online.