This is not legal advice, but we have had a similar issue, and this is the way it works.
If an owners corporation becomes aware that an owner is using either their lot or part of the common property in breach of regulations, or in an illegal way, or in an unauthorised way, they are then on notice that this is the case.
The question then is whether the OC has a duty to tell the owner not to do it, or take steps to prevent them doing it. This will depend on the circumstances.
In your case, if the OC is aware that there are unapproved modifications affecting the common property that may be a safety risk, and may in fact compromise the safety and/or integrity of the building, then they should take steps to rectify the situation. This may include requiring the owner of relevant lot to reinstate the common property. If they do not, they may be liable (and refer below re insurance).
As regards insurance – as a general rule, where the OC is aware of something like this, it can void their insurance either in whole or in part. A contract of insurance is uberrimae fidei – a contract of utmost good faith. Anything pertinent to that contract must be disclosed. Where the OC becomes aware, they have to disclose it because it is relevant to the contract. If the OC were sued for an injury resulting from the modifications, the insurance company could refuse to pay and then the owners would be personally liable.