Although from seeing a photo one leaf it’s difficult to diagnose a problem, here are our thoughts on your hibiscus plant:
1. This does not look like typical insect damage on an interior hibiscus. The most common insects on these plants are whitefly, aphids, or mites and this damage doesn’t look like what those bugs do. So it’s pretty sure we can rule out insect damage.
2. This also doesn’t look like disease damage. Fungal diseases cause yellow or dark spots, not what we see in this photo.
3. From what we see this looks most like contact damage of some sort. Was this plant recently fertilized? If so you might be seeing fertilizer burn if the plant was fertilized when it was dry or given too strong a solution. Were any cleaning products used near the plant recently such as window cleaner? If so it’s possible that the damage has been caused by such products hitting the leaves. Or was some other garden product sprayed onto the plant such as an insecticide or fungicide? Might something else have been dumped on the soil in the pot, or might a cat be using the plant for its litter box?
If you think this might be fertilizer burn be sure to water the plant really well by placing it in a shower or tub and running a great deal of water through the soil to wash out excess fertilizer salts.
Whether you figure out what caused these spots or not the treatment would be the same: remove the leaves that show these symptoms so you can monitor if the problem is still going on. Keep the plant in good shape as you obviously have been in the past – the other leaves we see are a good green color and look healthy.