Oil in the airbox

My experience comes from a DL1000. The missing foam keeps the oil blow by from getting into the filter box. They just disappear sometimes.

My box would get some oil in it but yours appears to be mixed with water or coolant. I am hoping a 650 fan weighs in because they have more blow by at high rpm. Currently, my gut tells me yours is excessive.

Since the bike is new to you, I would say first clean out that mess from the air box plus replace the breather sponge and then inspect after a good test ride. A tip over in water could possibly account for the current deposit in the airbox. The 650's have their breather hoses on the left side of the engine. If cleaning don't help, then as in #2 above, your need to look deaper into the cause.
Helpful info on breather filter / sponge shared in threads like Foam Filter under Air Filter, Breather box filter material, Breather Filter Part Catalogue - Air Box Part No. 6 , Is... and 2005 DL650 Need Help Finding a Substitute for Crankcase...

I agree with Gert the bike has been dropped, when these bikes get dropped on their side the oil makes its way up to the air box, your oil has water mixed with it.

Did the mix happen in the motor before the drop or was it in water when it was dropped.

I hope the bike was cheap because I don't think the seller was very honest if he didn't tell you and tried to clean it up, if he told you that's another thing.

Looks like dried mud all around, so I wonder whether that murky crap may well not have come through the crankcase breather and instead from a water crossing gone wrong. What does the outside of the air filter look like?

Agree w/Gert, best approach is clean, cover some highway miles and then re-check.

Also check to see if your coolant is low. Coolant and oil produce the result. Check the oil sight glass to see the color of the oil. If it is the same color as the oil in filter box there maybe other issues.