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I have a e350 mercedes 2006 with82000 miles on it. Last month I noticed about a tabelespoon oil

Also it had a message of brake service on the screen. I took it to the shop, and they changed oil and said no evidence on oil leak. The brake message was brake blob needed to be changed. Two weeks later, the engine light came up, took it to dealer. They replace the air pump injector, I guess. One or two times early in the morning, I smelled rubber burn smell. But I could not reproduce the smell and did not come back, so I ignored it. Yesterday again, I noticed about two tablespoons under the car. The leak is not right on the center, it is more towards the driver side area. I parked it somewhere else to see if we get more of a leak. No leak so far. However, I noticed some evidence of spotting on my garage floor that is not recent and very small. I am not sure what to do. It runs ok, the oil level is perfect. No temperature issue. What can it be? Should I wait to see some changes to take it to dealer or should I ignore it or should I take it now?

How is the brake fluid level in the brake master cylinder?

Don’t wait for trouble. Take it to the dealer. Ask them to again check for oil leaks.
Have the dealer specifically check if the rear camshaft plugs are leaking. This was somewhat common on the M272 engine, which you have.
Your car has a rather large belly pan underneath the engine, by the way. It actually does a pretty decent job of catching leaks.
More food for thought: The M272 engine uses a fair amount of oil. You CANNOT go the full distance between scheduled services without topping off in between. It will very likely be at least 2 quarts low if you try it.
This was objectionable to some customers, but deemed to be normal by Mercedes-Benz.
After you get the oil situation resolved at the dealer, I suggest checking/correcting the oil level monthly.
And only use the oil that Mercedes-Benz recommends, Mobil 1 0W-40 full syntetic, I believe.

Are you sure it is oil and not water from the a/c drain?

The spot of oil, could be some other fluid. Brake fluid, power steering fluid, transmission fluid, coolant, or even water. Dip you finger in it and look at the color, you can taste it and if it is coolant it should show some sweetness on your tongue.

If you don’t want to get your hands dirty, wipe it up with a paper towel and place the towel in a plastic bag and take it to the dealer to examine. Take picture of the spot on the garage floor and show the shop approximately where the leak is on the underside of the car. This will help the shop know where to examine and what to be looking for.

Some leaking from the steering rack is possible, there are transmission cooling lines from the radiator to the transmission that could leak. A loose clamp could allow some coolant to leak. Lots of possibles to check out.

Have someone check your left and right valve cover gaskets. And as someone said above, there is a pan under the engine so it take a fair amount to gather before you see it under the car. I have a 1999 E320 wagon with 175,000 miles and regular upkeep is the key! It is such a great car – worth the effort!