2005 Vauxhall Vectra Oil light coming on

I have a Vauxhall Vectra (2005) 3.0 v6 diesel. Car is second hand with only 28,000 on the clock before we bought it only been serviced once though. On long journey's (i drive 6 hours once a month) I frequently get the oil light come on. There are two oil lights according to the manual one has a wiggly line underneath one doesn't. I'm getting the one with the wiggly line which is a warning for low quantity. I stop check the oil (although engine is warm) there is always plenty in there. I know this anyway as I always check before going on a long journey. The first time this happened we took it to a garage and they said it's because it needs an oil change every 10,000. So we got this done and the oil light didn't come on again for 9 months or so. It came on last week doing the same journey. It came on twice, I stopped the first time checked oil, plenty in there turned the ignition back on and it remained off for 2 hours or so then came back on. Same again - stopped, checked oil light went off again and didn't come on again. What is puzzling me is what triggers the light? It's clearly not a sensor for low quantity, unless sensor is faulty? I thought it was just 10,000 mile trigger that is reset by a mechanic when he's done an oil change, but this can't be true either as I haven't done 10k since last oil change. Could someone please tell me what the trigger is, so I can sort out the underlying problem. Thank you very much for your time.