I wouldnt worry about it and don't even bother disclosing it unless it comes up in an interview. You weren't arrested, fined, or formally charged.

If the application or the interviewer asks a question like:

"Have you ever been under investigation or questioned by CBP for a potentially illegal entry into the country", then you should check the yes box.

To my knowledge, nothing that you did was illegal based on what you said. It might have resulted in a flag on your passport (perhaps to let agents know if it happened again), but I wouldn't worry about it.

If it does come to light in a background investigation, answer the questions truthfully and explain that it wasn't a big deal and you were released by after a detailed review of the flight documents and circumstances. No biggie.