I've returned, thanks for posters, I've read them and I'm glad most don't think I'm a hysterical mother overthinking.
My parents are in the same hotel, a few floors down and their balcony door locks so me and Dd bunked in with them for the night (my dad was NOT pleased but I seriously was not able to sleep). We've all had a laugh about it now. Waited for reception to open in the morning and complained and had my room switched for tonight to one with a locking door.
Like one poster said, although the risk of a stranger taking Dd is very small (and my fear was probably fuelled by awful cases in the media like Madeline MCcann) the potential consequence was too great for me. If she had been taken, I don't think I could have said to myself "she might be gone, but at least I slept well!". A seriously disturbed night sleep was worth it for me in the end, so I could be sure I did everything to keep her safe.
I think until dd is older we will be avoiding hotels with balconies where we can! I just couldn't be sure that last night was not the night she decided she wanted to see what was in the balcony (which she has ignored up to now) while I nipped to the loo or something.
It has raised an interesting idea in my mind though. I live in a city and city etiquette is always to lock your doors and the idea of sleeping with an unlocked door onto a communal space really scared me! But it seems a lot of people don't have this fear in their life, it's just interesting to see the difference in a few peoples attitudes.
One of the huge worries for me is that there seem to be a few stag dos in the building and I just kept thinking what if one of them came in on a drunker date or something? Even something as ultimately harmless as that would have terrified me and DD.
Anyway. Thanks all. And for goodness sakes, lock your doors!!!!