I had the same issue with the second plant and to this day it stayed a mystery to me. I think I posted about it here and another user said that the same sedum plant she had, had the same scar marks appear from nowhere. It was early spring I think, there were no harsh sun on warm weather, both plants were indoors. Some guesses were sun/lamp damage but it wasn't in my case, the other one was damage by pest - also didn't noticed anything, and the third one was damage by other animals, but i don't own any. Maybe oedema? No idea.

So.. In the end it just grew out, new leaves didn't have any scars. ¯\_(ツ)_/¯

Thank you Nik and Dominyka for your feedback. She does not use grow lights, and so I feel like it can't be sunburn because like I said it developed while the plants were indoors for the winter. I feel like it could be a pest, but she has probably 50+ succulents and these were the only two that got this damage.

I did look at the possibility of edema. I just learned that term yesterday when doing some research on this issue. I suppose it could be that. We are just perplexed.

Thanks again!

There doesn't seem to be any signs of infestation or large pest chewing on the plants.. My guess is similar to the folks above - sun burnt. Sometimes, intense window lights can magnify the heat on certain spots of the window, so the change of increased light and heat might be the problem. I have grow lights, you can almost NEVER get plants burnt from growlights, unless you don't water them and let them die..

Sun burnt marks are very common during seasonal change especially when it gets warmer in the summer with longer day light. The best way to avoid is set up a shaded netting or have plants placed in a green house to adjust the light change. If any thing, new leaves will grow out find, so you shouldn't worry too much.

I don't believe it is oedema. I don't think it is sunburn either - but possibly. At least on first plant.

Ppl posted photos of similar damage, and it is likely caused by something chewing on it while still younger. Those 'bites' have scabbed over. As the plant grows, rosette opens up more and damage is more visible. You can see it is sort-of regular. Very young growth is usually most juicy/succulent, and it is often first to be chewed on. One doesn't have to see the bug or pest for it to happen...and just because other succulents were not damaged, it doesn't mean it didn't happen to this one. I actually witnessed this to happen. As Dominyka said, those leaves will eventually grow out, and plant will look better.

April

If you don't mind here is a suggestion: hope those metal tins have drainage hole(s). If not, they should be drilled or poked in. And if possible, top up the potting mix so the plants are not so deep inside of them. Metal heats up more than terracotta (didn't compare it to plastic); and plants are even 'hotter' if they are deeper inside. And it restricts air circulation too.

Bernard and Rina -

Thank you for your help and advice! I never thought of that Rina, regarding the pests. It could have very well been that. Plus there are creatures all over where my mom lives that don't necessarily eat succulents, but may have tried biting into one to see how it tasted. Or knocked it over. Squirrels, her cat, birds of all sorts, etc. She just keeps them on her patio during the summer months.

Rina, I'll give her the advice about the metal tins. She's much more relaxed about her succulents than I am, but I try to encourage her to practice better drainage and soil, but she often shrugs it off. I'll see what she has to say to your advice :) She keeps succulents in everything from these old metal coffee cans to drift wood to vintage baby shoes. Hah. Thanks again!

April

Squirrels are 'famous' for munching on juicy plants. I have chipmunks here and they are very mischievous. Digging in everything - pots or ground, damaging whatever they get close to. I have seen one getting into my Lithops, and when I tried to chase it away, it did run away...with 1 of the plants! I get bites on many plants too, and you can see the teeth marks. They will dig flowers, vegetables - anything.

Try to convince your mom about drainage holes if possible. It will help some, even if she doesn't improve soil - and that would be a great help.