Third type conditional

[navi]

1-We don't know if they left the building or not, but if they had been told to leave the building they would have left it.

I was taught that the third type conditional was always counter-factual, but could it work in cases when one does not know what the facts were?
In the above sentence, the speaker does not know whether they left the building or not, nor whether they were told to or not. He is merely stating that if they were told to leave the building, they had to leave it. Is the sentence acceptable in such a context?

Gratefully,
Navi.

[Alan]

All 3rd conditionals are, by definition, counterfactual.

I see no problem interpreting this sentence: the first coordinate clause directly asserts our lack of knowledge as to whether they in fact left the building, whilst the subsequent conditional matrix indirectly asserts merely that they were not told to do so (whilst imagining the consequences of the reverse situation).

Naturally, one assumes that they may still have left the building even without being instructed to do so (i.e. for some other reason), and so the situation is simply left unresolved.