It's always crashingly dull derivatives or some other mundane financial industry rubbish.
Or, on one occasion in the Holideck lounge in T4, some negotiating tactics for investors dealing with a small company that a friend of mine was heavily involved in. A quick, quiet phonecall relayed that conversation to my friend, and we boarded while the negotiations were due to take place.
After landing, I must admit to projecting my voice unnecessarily as I phoned up to ask my friend how the negotiations went, and if my overheard information was useful to him.