From where did you download Electrum?
Did you ever receive any popup message telling you to download an update from the wallet?
Everything seems to be fine from the side of the ATM operator since you received those coins. It looks like you used that wallet 3 years ago so I assume that you have generated it on your own. Was your PC powered on when you received this transaction? There was a bug which was fixed in Electrum 3.0.5 which allowed malicious websites to steal coins from passwordless wallets.
Well, no popup windows, nor any strange behaviour from my PC...
And of course, the downloaded wallet is from official Electrum.org.
I´m pretty sure that my Win7 Dell is very well protected... and it was powered on, I was controlling it remotely from my phone. Well, I assume that the most probable case is the bug-thing. Good to know...
Never ever gonna use Electrum anymore.