Thanks for your reply!
I also reopened the wallet, and accidentally expanded the "Used" subfolder in the addresses. And there was the original address.
Due to the way it was lined up with the other 5 receiving addresses, I thought it was the 'folder' for those 5 addresses.

Are you suggesting to use a different address every time for BTC in general? Or for users of Electrum?
Would you say the same for major vendors online that accept bitcoin?

In my case, the coins I sent out were to a mining pool, and dividends will be paid back to the address they were sent from. So I was needing to keep this address for a while!
Thanks.