Grammatically speaking, there is no indefinite article in Koine Greek, but its place is often supplied by the indefinite pronoun (any, a certain, a mighty angel).

In LXX, ἄγγελος Κυρίου can refer to THE angel of the LORD. Judges 6:22: 
>When Gideon realized that it was the angel of the LORD [ἄγγελος Κυρίου], he exclaimed, "Alas, Sovereign LORD! I have seen the angel of the LORD [άγγελον κυρίου] face to face!"

But it can also refers to an arbitrary angel of the LORD. Zechariah 1:12:
>Then the angel of the LORD [άγγελος κυρίου] said, "LORD Almighty, how long will you withhold mercy from Jerusalem and from the towns of Judah, which you have been angry with these seventy years?"

So, the phrase ἄγγελος Κυρίου is ambiguous in the OT. 

Yes, ἄγγελος κυρίου in the NT (e.g. Acts 12:7) ambiguously refers to the angel of the LORD in the OT.