What would be the proper adjective to describe an 'open' padlock?

I'm trying to describe an 'open' padlock hanging from a latch. ['open', so a character can lock someone else in the following paragraph]

Is 'open' the right term?

I've also seen 'unsecured', 'unlocked', and 'free', but I'm not sure which term is of common use in American English.

An open lock is what you want. That is what the next character will need if they are to fasten the lock shut later.

'Free' is not in common usage much in American English. Though 'unlocked' is a good description as many locks can be shut without being locked. These appear locked or latched but are unsecured. Your goal seems to be a lock that on sight is clearly open and unlocked.

Open is the right term.

That said, if you wish to get really technical and pedantic and all, you might want to consider "unlatched."

As in:

An unlatched padlock hung by the shackle from the loop.