oil warning horn question

[LEVERETTE5]

I have a 2000 OX66 200 yamaha. This weekend while fishing my oil horn went off. There is a little circle on my gauge with a line thru it that was flashing at the same time. Both my main oil tank and my small tank on the motor were full of oil. I pulled the sensor out of the small tank and moved the float a few times put it back in and it was fine for about an hour and it did it again. I have looked in my repair manual and I did notice that there is a washer that should be on the bottom of the strainer that the sensor slides into on the small tank that is missing. Im not sure but it also appeared that the same strainer was not properly on the sensor( the repair manual has a picture of how it should of been. can any one tell me how to trouble shoot the oil system and what sensor would make the horn go off.the parts manager at pompano marine center said that the sensor in the small tank on the engine is the only thing that will activate the horn,is this true? Thanks Keith