Concur with those posting above, if there's no explanation for the high oil level on the dipstick, best to not start the engine; instead drain a pint or so sample of the oil out from the oil pan drain plug, and take it to a mechanic for a visual inspection. If there's water in the oil, since oil floats on water, the water will tend to go to the bottom of the pan, so if there is water/coolant present in the oil, it should be evident in the sample.
No water in the oil? If the last oil change, somebody simply made a mistake and put too much oil in -- like they forgot to drain the old oil first say -- that needs to be corrected asap too. Running an engine with too much oil in it causes all sorts of problems, including the possibility of causing leaking seals which will be costly to fix. It could cause a false sensor reading too I expect.