I had initiated my wallet using Electrum, deposited smaller amount of bitcoins into it (last time on April 3rd 2018) and deleted wallet. Yesterday I tried to restore wallet from the seed, and found that there was transaction on 17th November 2019. I *might* have used Electrum during 2018 or 2019 (I had mac at the time, now I am back on windows, I also vaguely remember there might have at some point been Electrum for iPhone available so I might have used that to check my balance). I do not see any other way of stealing my coins but by compromised Electrum wallet either on Mac or iPhone.
1. You should have started your own thread for this.
2. You don't have to start Electrum to deposit money "into it". Electrum only handles the keys to your coins, the coins are not in Electrum, they're on the "network".
3. I see other ways too to steal your coins:
* seed stored in mail, cloud or other unsafe places, including your desktop; also is it you who has generated the seed?
* other malicious software accessing files your Electrum or iPhone or Mac may have not been deleting (I don't know if/how possible that is on iOS though)
Of course, malicious wallet is also a possibility.