Ignition stuck on.. (flat battery)

[FuZzBoM]

All, here is an odd one for you.. I went out for a ride yesterday and when I got home put the bike in the garage as usual. Turned engine off and went in the house. This morning I go into the garage and see the green neutral light on??!! First make sure I hadn't left the keys in it by mistake... No Keys in sight. I think well maybe I had not fully turned off when key was taken out. go and get keys. Turn on, still light, turn off still light. move round to lock the bars and still light on!!
whilst in the steering is locked I press starter and I get a lumpy starter motor noise (no surprise as battery flat)
The only way I could get the light to go out was disconnect battery. stuck it on charge and did what I needed to do in the garage.
Anyway getting back to the bike I then put the leads back on and no neutral light.. put keys in on position and on ok. now keys in off position and now off. (also bike starts ok and stops ok on the key)

A little confused and tbh a little worried about leaving the bike anywhere if it does it again.
My thinking is that I MUST have turned off or the engine would have probably kept running when taking the keys out?!

51 plate thou by he way.

thanks,
Fuz