I just noticed that I always automatically lock the toilet / bathroom door, even when I'm by myself.

As far as I know I always have, but I've also not lived by myself very much - always living with family or housemates, and now with my own wife and kids. I did live alone once, but only for about 3 months.

So, what do you do? And do you think it's due to time living alone or with others?

Edit: reading through the comments, this brings up a secondary question - how do you verify a toilet isn't occupied?

I think the four most common ways are:

1) Knock 2) Try the handle 3) Say something, such as 'anyone in there?' 4) If it's a public loo, look for the red marker on the lock.

Which do you do?

If at home I tend to try the door, I guess because I assume everyone locks it. So unlocked means unoccupied.

If I'm in a public loo I'd look for the marker first. It always annoys me when people rattle the door when I'm on a public loo!