A few days ago, I gave a summary of Gordon Fee’s discussion of πνευμα in Paul’s letters from his book, God’s Empowering Presence: The Holy Spirit in the Letters of Paul.
Interestingly enough, I came across some similar stats with regard to the phrase “ἐν (τῷ) κυρίῳ” (in the Lord). This phrase gives us evidence that the occurrence of the article in such constructions has little to do with the definiteness of the noun. Of the 49 occurrences of this phrase in the NT (and all except one are in Paul’s letters), only Ephesians 1.15 has the article. And as far as I can tell, never is Paul referring to just any Lord. He’s talking about Jesus the Messiah.
Now this doesn’t prove that every instance of “ἐν πνεύματι” (by the Spirit/in the Spirit) is definite or indefinite. But it does show quite clearly that the presence of the Greek article is less significant in adjunct prepositional phrases for determining definiteness.