Ford Focus Zetec 1.6 57 plate Bonnet Open warning?

Hi everyone I have a Ford Focus Zetec 1.6, 57 plate. Last night the alarm suddenly went off and when I check the dashboard trip computer displayed: BONNET OPEN and a red warning light was on. I then opened and closed the bonnet several times but the problem was still there.

Then I googled it and found this forum, following other's experience I checked the fuse box under the passenger side glove box but there was no unplugged connector. To make sure I even unplugged the two connectors by myself and re-plugged them back. But nothing was solved. I tried my electric windows and other things they all worked properly.

The weird thing is that after ruining my night with 4 alarm going off, the car itself solved the problem: when I turned the ignition on this morning the problem was gone! No bonnet open warning anymore! I thought that was the end but just 1 hour ago the alarm went off again and the BONNET OPEN alarm was back with it! Before I went to a mechanic or Ford, can anyone kindly give me some advises which may help other than checking the connectors under the fuse box? Many many thanks!