I believe there is a switch with 7 contacts (one for each gear and one for neutral). Either its contacts are failing, or, more likely, there's corrosion in its connector. I believe it's located low down on the right-hand side of the crankcase.
Can't be any more definite, my AS (AE for the USA) doesn't have a switch for gear indication.
My '15A is starting to get flaky indicating 5th gear. Seems more likely to go blank when beginning the day in colder conditions and then gets more reliable as the bike gets hot and the day goes by.
IIRC there's a couple other threads and reports of the gear position indicator needing replacement.
On my '07, I had to replace the "switch". Actually a circular flat insulator with six contact pads. A pin in the transmission made contact with one of the pads to indicate the gear. My neutral light was flaky for a while and then stopped working altogether. Connector was OK but there was a broken wire in the potted part of the device that would have been difficult to repair and reseal.
On my 2016 this week the gear position in the screen was intermittently loosing 1st and neutral. Thanks to twowheelobsession on you tube it i took out the doo-dad on the left side as he directs cleaned out the area. Now it works perfectly. Also on my 2016 there are allen head bolts and no crappy soft phillips head screws to worry about.
That is the part I replaced on my '07. I had unplugged the connector and checked the continuity between each of the contacts back to the plug. All were OK except for the neutral. Cleaning it up didn't help. I stripped back the wire to the neutral connection and there was no continuity from the pad to just outside where the wire went into the potting so it couldn't be fixed easily. Part (for a Gen III) is $42 - #6 on this fiche. Check it out before you buy might be nothing more than a dirty connector.