I periodically check flight status on Delta's website, and normally they run two daily flights DTW-LHR: Flight 16 departing 6:16PM and 18 departing 10:37PM.

On either Monday or Tuesday, a third flight was listed. The flight number was four digits (believe began with 9), with scheduled departure within 5-10 minutes of 16. This flight used a 767.

This "extra" flight is not on offer on today's schedule (July 18).

Any idea if Delta ran this flight strictly for heavy general summer demand, or if it was a one-off prompted by, say, a large group enroute to a convention?