Anybody on your network but you? Friends, family etc?

Password is transmitted in clear text & very easy to obtain if on the same network.

I've never seen a pw change itself, have you looked in logs for logins etc they are kept after a reset on my HH?

No one other than my immediate family has access to the wifi. No one other than me knew the admin password. Those who have access to the wifi would not have a clue what an admin password was, let alone how to change it.

I will have a look at the logs when I get home later, I had assumed they would be deleted but hopefully not.

Got any home plugs on your network? Are they encrypted ? Could a neighbouring house be connecting to your router in error - although how they'd get into the admin part... ?

You can override the password on a HH

Nope, no home plugs.