Very much case by case.
I used to live in a street that had tons of stuff left outside of buildings (think chairs, shelves, mattresses etc) as tenants moved in and out a lot.
I got loads of useful stuff from that practice.
You just wait or ask around a bit before taking. Realistically a sofa left outside on a London street for a few hours is _at best_ temporarily awaiting disposal.
I found a sign that said ‘FREE - WORKS’ and was putting on things for amusement until I realized that might be a very bad idea.
When taking something that says ‘free’ how does one verify the sign was written by the owner?
As for the first, It’s the same as someone uploading a stolen photo with a CC0 license, i. e. none. If the true owner finds you, they are highly likely to be entitled to get their property back.
As for criminal charges, a reasonable assumption of permission should usually work as an excuse. This will depend on the specifics, such as the item’s value, local customs, etc.
I actually found a rather valuable chair (the rietvelt) on the street, and did leave my phone number because I could not quite believe my luck. The previous owner called and told me to enjoy it.
You could protect yourself from inadvertently depriving the owner by leaving a note - "thanks for free stuff, [email protected]".
In England: Probably, yes. You have appropriated something that belongs to someone else; you intend to permanently deprive the ower of it. The only other thing needed for theft is for the appropriation to be dishonest.
You know the couch doesn't belong to you, and there isn't a sign saying "Free couch", so there's a strong argument that this is dishonest unless there's a sign on it saying "free couch".
Here's an article about stealing rubbish: https://www.bbc.co.uk/news/magazine-13037808
Here's some commentary about a story of people stealing out of date food from a bin, being charged for it, and then the CPS dropping the charges: http://ukcriminallawblog.com/stealing-from-a-bin-and-the-vag...