Grammar of present perfect continous of sit and stand

Question

I've often been seeing phrases like the following lately:

s/he has been stood

s/he has been sat

used instead of the present perfect continous i.e.:

s/he has been standing

s/he has been sitting

It's usually only used with sit, stand or similar verbs of position. Now, I'm not a native speaker and I've never come across this until a few years back, but it doesn't stike me as a rare occurence by now.

My question is: What kind of idiom is this? / Where is it used?

PS: If this construct is ungrammatical and you want to point this out, please do so in a civilized way. I mean, I guess it is, but I'm not into the kind of disdain that regularly comes up in that kind of discussion.

Edit: I was ask to provide context, which I see is necessary as some commenters pointed out correctly that the first construction is of course perfectly grammatical as a passive construction. I'd like to stress that this is not what it's used for in this case. It is really used equivalently to present perfect continuous. It has also been mentioned by one commenter that there exists a confusion between sit/set/be seated and lie/lay ('lie' could be another candidate for the use I described, but I'm not sure), but I'm uncertain if this has anything to do with this, especially as there's no such confusion with 'stand'.

Two exemples would be:

Bob, who has been stood outside the door smoking, comes back inside.

The baby grand piano has been sat in Fiona's front room for a grand total of three months now.

Answer 1

Your first example, has been stood, is grammatical; but it is not equivalent to has been standing. It is the ordinary passive construction, BE + past participle, employing stand as a transitive verb, as when we stand a vase of flowers on a table, or stand an unruly child in the corner.

In Bernard Shaw's work we frequently find that a conventional platitude has been stood on its head.

Your second example, has been sat, can be grammatical only in certain idiomatic uses, because sat is the past participle of sit, which is an intransitive verb, and intransitive verbs cannot ordinarily be cast in the passive voice.

(ADDED:
But as Peter Shor points out, sit is used in a transitive sense of cause someone to sit [at a particular place at table]. To my ear the perfect passive "He has been sat between the Governor and the Mayor" grates; I'd say "He has been seated"; but Peter accepts it and I accept him as representative of "Standard" users. ... And there's also the prepositional passive: The candidate has been sat on by his appalled campaign advisors.)

However, it is not at all unlikely that you should hear the construction. Even fairly well educated speakers frequently confuse sit and its corresponding transitive set, just as they frequently confuse lie/lay and rise/raise.

Somebody's been messing with my stuff. My hammer's been sat on the shelf instead of hung on the peg where it belongs.

In colloquial English these pairs have practically converged.