I used to have a Nissan Micra that went in for a service and came back with an oil leak. I thought it was the rocker cover gasket but took it back to the garage where it was serviced. They had a look and fairly quickly realised that they had left either the plug or some sort of bolt out. They put it back, refilled the oil and off I went.

In short, I think your garage is probably telling fibs. They either did a similar thing or possibly didn't screw the oil filter back on properly (if they changed it). Did you have high fuel consumption before it went in (fuel line)? Did you have any symptoms of HGF, such as white smoke, boiling coolant or mayo under the oil filler?

All MB models have an engine oil sump drain plug, but not all have a dipstick tube .

Like some VW Group engines - perversely my 3.0 TDi VW has an unused dipstick tube, but no dipstick - electronic level gauges are great until they fail.

Making yourself a dipstick would not necessarily involve too much rocket surgery.

If its a straight stick you can use a meat thermometer (gives you oil temperature).

I use a magnetic pickup tool.

A LOT more rational and informative than a magnetic drain plug, but never an OEM fitment AFAIK