Why is their Molybdenum in my engine oil sample?

Molybdenum is usually 3 potential sources.

1) molybdenum grease contamination but this would be cause lots of wear and potential be showing an increased visibility too.
2) molybdenum from ring wear. Some engines have on the top ring a molybdenum coating to cope with high temperatures. You would need to check with the engine manufacturer if this is the case.
3) molybdenum as an additive. It is molybdenum disulphide, which acts the same as graphite would as a solid lubricant within the oil and has some synergistic effects with other anti wear additives. It used to be a couple of oil manufactures that used this additive but the the levels to which it is being used and the number of manufacturers buying molybdenum containing additives is increasing that it’s use as detecting potential ring wear is now limited.

The best thing to do when assessing is to compare to the new oil to see if it is roughly the same. It may be slightly higher in the used lubricant as the oil oxidises and some of the lighter fractions of the base oil are volatilised in use leaving behind more concentrated additives. If there is a significant difference then it’s worth establishing if the oil manufacture has recently done a formulation change so that the used and new oil could have different levels, or if the oil is a mix of multiple lubricants as the oil has been changed by top up rather than flushing the old oil out.