If the unit has been open for inspection, you have as much right to see what’s going on there as anyone else and report back accordingly.
Meanwhile your strata manager is being unnecessarily overcautious. The EC should ask them to send a letter to the vendor’s estate agent and to their solicitor and conveyancer, with a copy added to the scheme’s records, that they believe there have been unauthorised renovations and additions to common property (without specifying what they are) and that if this proves to be the case, they should either be removed or responsibility for them will pass to the new owner.
If the vendor objects, then you should ask if you can have a site inspection to verify that there have been no unauthorised renovations after which you will withdraw the letter immediately.
Your strata manager needs to be more proactive – if she’s not prepared to do this, your next task should be to find someone who is prepared to do their job. her lack of action will cost your owners corp in the long run.