Answer: This is perplexing. Foliage damage such as you describe is most often the work of an insect such as caterpillar, beetle, grasshopper, leaf cutter bee and so on. Sometimes foliage is torn by wind, also. Certainly these are more likely to occur outside. But I think there must be an insect -- perhaps active only at night as some do -- and on an indoor plant just one insect can actually do a lot of damage.
The only other thing I can think of is that since it is indoors and has never bloomed, it may be in far too little light on the windowsill (it is a tropical plant after all and needs high intensity light all day long such as we receive in summer) and also overfertilized with nitrogen. This would lead to very thin, overly delicate leaves. However, I do not think they would spontaneously tear.
I would suggest you take some photos to your local professional nurseryman and see if they can help you identify the culprit by seeing the pattern of damage. Based on knowing that, you can determine how to control it.
I might also mention that the blooming issue may be related to lack of light, oversupply of nitrogen, and incorrect pruning. This plant blooms on mature wood, so it needs to be allowed to grow naturally and form flower buds. Pruning is done after it has bloomed.
If you have very limited space, this may not be the best plant for you.
Most hibiscus are rangy shrubs rather than tidy container plants, and so they are chemically treated at the grower to remain compact. Eventually this wears off and they begin to grow normally and can grow very big. Pruning is done after a bloom flush, either selectively removing the longest branches or pruning the overall plant back quite hard. It takes the plant several months to recover from this treatment and bloom again. Northern gardeners will often prune it back quite hard in late winter then hope it regains strength enough to bloom the next summer. They will also set it outside for the summer so it can get ample sun.
I hope this helps.
Q&A Library Searching Tips