Unfortunately, not so mechanically minded...

They may of spilt some oil when refilling, was the oil gold in colour
Or when they replaced the oil filter they may of spilt old oil as it can get a bit messy as the filters are normally in a bugger of a spot.

Has it got oil on it now since you cleaned it maybe just check to see if the oil filter is on tight enough should not be able to move it with your hand, It’s the white, black or blue thing on the side or bottom of the engine, You may have 2 of them.

Possibly the sump plug has not been done up tight enough, look for oil around the sump on the motor.

Is it a Auto, Possibly transmission fluid return or out line to the cooler, If it was Auto transmission fluid it would have been most likely red in colour and smelt a bit funny and had a very thin feel to, as it it’s not oil but more of a Hydraulic fluid.

Cant really recognize the pic, but oil does splash around a bit during a service, even worse in a diesel cause it´s black oil.
Just spray it with degreaser and rinse off. Then check it a couple days later.

Ulises

Take it back to who ever serviced it.