On the NG it is not uncommon for the spoiler lever to be slightly out of stowed position. This will trigger a take off warning the moment thrust is set. You close the thrust levers, give the spoiler lever a push, then take off.
I would never write a report about this
This is exactly the kind of stuff Huck is talking about with the feds in the U.S.
If you had a takeoff warning and didn't document it, even though you know what caused it and corrected the problem, you might be hanged these days.
You'd better contact dispatch, maintenance, flight ops, and write it up in the logbook. Then decide whether it is a discrepancy or a comment, close the writeup somehow with corrective action like 'cycled spoiler lever, subsequent operation normal' and get a bunch of codes and employee numbers. And for CYA purposes, within 24 hours file an event report, an ASAP safety report and maybe a NASA form.
And, since you may have rolled a down the runway a little, make sure you have data for takeoff from the next intersection and run the numbers again just to make sure.
Or, roll the dice since you are on the runway at PVG with twenty planes behind you for an afternoon departure.
As Dirty Harry said: "You've got to ask yourself one question: 'Do I feel lucky?' Well, do ya, punk?"
The company 'self-discloses' these events to the FAA and like Huck says, they go through QAR and other data to see if it matches with the logbook and event reporting. Remember when we were told that all this data collection was totally anonymous and would never be used for enforcement or punitive purposes?