In the 70's I had an M2 and an M3, usually with the 90 or 35 Sumicron attached.
One day I saw something like this, and I realized that I had carried the 90 camera around in my car for a couple of days without the lens cap.
Apparently, it sat for a long enough period, pointed up (following me here?) toward the sun.
Remember the childhood trick with the paper and the magnifying glass, and the sun? You bet, burned a hole (nice clean round one) in my curtain, based on how it was focused, etc. One nice thing about SLR's, there's a mirror to prevent this.
I never went without a lens cap again.
Might this have happened to you?