[binty]

Hi we had the same problem and cured it. Based on a 2014 D4.
The left hand catch has the micro switch.
Basically the two catches are mechanical affairs and passenger side one has a micro switch as well. This fix took about 15 mins.
It was lubricating the catches that caused the problem, as the lubricant got into the micro switch and prevented it making contact, hence it registered bonnet open.
This is what we did.

Temporary cure,
Remove the passenger side headlamp, one x tor’x screw, lift out the bonnet bump stop, (I love the way Volvo thought to make this removable), then you can pull and rotate the headlamp out and remove the wiring plug.
You will see that the wire from the hinge has a connector to the wiring loom at the side of the headlamp. It is actually at the wrong side of a panel for you but the plastic lugs stick thorough and can be squeezed with pliers and pushed towards the centre of the car.
You should be able now to separate the connector. The pins are on the catch side, the sockets on the loom side which is good.
Short out the two sockets on the loom so it forms a continuous loop, (this is all the switch does, it connects these two wire together), I used a small electrical spade connector and bent it to fit. Your bonnet open light should now be off (permanently).
Cover the loom connector with tape and cable tie the wire so it doesn’t glob about.

Permanent fix.
Look into the passenger side hinge with a torch from the top. You should be able to see the micro switch as it has a tiny red button on it, deep down, on the headlamp side.
Spray the micro switch with electrical contact cleaner, or brake and clutch cleaner, but nothing oily.
Toggle the micro switch button on and off numerous times to get the cleaner through it.
I used a small hex key as it was the right size to press the little red button.
Now leave it all to dry, for several weeks and repeat as needed!!
You can test if the micoswitch is working by putting an electric meter set to resistance across the pins and toggling the switch, you should see it change. Alternatively just plug it back together and see if it works.
When it is working again, remove the headlamp and clip the connectors back into the panel.
If it never works you can get used catches from eBay, or new micro switches from electronics retailers, but it looks to be a pain removing the whole lock mechanism which is tucked behind the bumper.

Our problem occurred after lubricating the locks, it took 3 weeks for the micro switch to dry out properly, it has been fault free for months now.
It taught me to be very careful where I lubricate the catches and not to spray liberally.

Good luck and let us know how you get on.