I was away from my W212 for around 2 months, during this time I had asked my uncle to drive it, I believe he only drove it 3-4 times and had it washed once after a sand storm since it became quite dusty. Prior to leaving the car, the headlight was completely fine and I didn't see anything like this.

I returned and noticed that the right headlight has developed these strange lines all over the lens and the top has started to yellow a bit. Is this due to moisture or condensation or another issue?

I have had the car washed twice since I noticed this, and despite being sprayed with high pressure water, I did not notice any water droplets inside the light housing so I'm at a bit of a loss as to what could cause this.

Pictures of the affected headlight:




And for comparison, here is the other light, completely normal:

 

Lot of glare on pictures, but that looks like common plastic discoloration due UV exposure.
Was the car park with this side more toward the sun, than the other side?

 

I looked at your pics: it is clear that L & R look like they are different age: old vs. new.

The problem here is the OEM HL are known to be $5,000. USD Ea.

Perhaps you can try polishing the plastic with specialized compound. The plastic used to manufacture HL gets discolored by sunshine. So there are many polishing kits available such as "3M" brand around $30.

Water has not got in yet... now is a good time to RTV silicone the HL rear connectors. The tiny wiring seals expand with age, helping channel water straight in to pool where electronic control is located.
✌️

 

Parked in a closed garage for the first 5-6 weeks and then in a partially covered driveway with a car cover on it. Even when I daily drive the car, it's pretty much always in covered or underground parking.

I don't trust myself enough to do the polishing. Do you think it's worthwhile to reach out to any places near me that do headlight restoration?

Aside from looking weird, would it affect the actual functionality of the lights in any way if I just let it be?

 

Thing about polishing the HL plastic covers is it only last about a year.

Get it done once to make this look smooth... results may vary because part of these defects may be internal.

For me, cosmetics is what adds caracter... who cares?