Given probably 99.99% of people would take an "are you hiring?" question as a very heavy implication that you'd be interested in working for that company and you explicitly make clear in your question that you aren't and the definition of misleading being
giving the wrong idea or impression
I think it's pretty safe to call that "misleading" yes.
Given that you used to work there it's not a stretch to imagine your former co-worker having a conversation with his boss along the lines of:
Remember GrandFleet who used to work here? Well he asked me the other day if we were hiring so I think he wants to come back. Would you be interested?
And if that happens you've wasted not your former co-worker's time but his boss' as well.
could this hurt me in any way?
Given you say you want to work in the industry (but not for that company) a decent reference or recommendation from this company might be useful yes? Well in that case this could definitely hurt you - misleading someone as to your intentions tends not to lead to people thinking favorably of you.
The irony is that being deceptive here literally doesn't benefit you in any way - you'd likely have gotten much better information regarding your actual query if you'd said something like:
Hey [former co-worker], I'm thinking of moving back into the industry, I'm not interested in coming back to ACME Corp at the moment but I was wondering if you'd heard anything about how the industry in general is going?