Oil light on this morning

Got my R850gs out this morning to head off to the bike show, got as far as end of the road and noticed oil light lit up. Turned round and back home on drive checked oil level. No oil in sight, very strange as I'd only topped it up last outing which was only 140miles.
Topped up to upper side of sight glass, tried again, light went out, but by the time I'd got off the drive it was back on again.
Not Happy......
Engine sounds fine....for a dumptruck engine anyway And it's only done 27k.
Hard to tell if its smoking as it wasn't warm enough and was still pluming like a cold engine does this time of year. No sign of any leaks, no oil on garage floor etc.
What worry's me is where did all the oil go having topped it up and only done the 140miles. I was in a group and I'm sure someone would have said if I was chucking it out the back, especially as our Tail end charlie has a HD that always smokes and gets the P taken for it all the time (Will talk to him to make sure)
I did give it a good scrub after the last ride, so I wonder if I got some water in the wrong place causing a short or something??
Any suggestions for what to check first?
Needless to say, I had to fall back on the totally reliable bike that I also own, my Blackbird, to get to the show

Oil pressure switch sounds like the culprit!

Best check before you set off after its been standing. If you start bike or only take a short ride it can "dissappear" for a while. If you park after decent ride and check after it's been standing you usually get the best readings. You could probably "lose" the top up you have done by starting and stopping the engine very quickly.

PS Oil switch +1

Wet sender connections near the sight glass. Can make the oil light come on esp if you use a pressure washer. Hope you haven't got too much oil in now!! Run it hot enough so that oil is circulating in the cooler, side stand for a minute or two on to the main stand.....leave it for a while and check level.

oil light

pull the lead off the oil switch turn on the ignition if the light works when taken down the road for a way or stays on all the time anyway it's a fault on the wiring between the switch and the light, Done it, been there and is a bit of a frightener.
dave GS.