In your original post, you said that “Everything still tested grounded”. I took that to mean that you checked continuity between the grounds and the grounded conductors. If you didn’t, I don’t understand what you mean by “Tested grounded”. Please explain what you meant by that comment.
No offense but I suspect that you may not understand the concept of “Isolated”. In the electrical world, “Isolated” means not electrically continuous. Transformers, for example, all fall into two broad categories, isolation transformers and auto transformers. The transformers that serve buildings are isolation transformers. They are called isolation transformers because the primary is inductively coupled to the secondary. The two sets of windings are insulated and isolated from each other. If you were to check continuity between the primary and secondary with a standard DC tester, there would be no continuity. There are also devices called intrinsically safe devices that are used for isolation. They are used extensively in places like natural gas pump houses. Unfortunately, the term “Isolated” is one of the most misused electrical terms by home inspectors. Being insulated, separated, or segregated at a particular location does not make conductors isolated. They are isolated ONLY if there is no electrical continuity between them.
The first thing to do, therefore, would be to check for continuity. If there is none, then yes, they are isolated. However, if there is continuity, then the grounds from the sub-panel circuits are mechanically connected to a common ground elsewhere. Probably, that common ground is another EGC elsewhere in the system.
When electricians wire a house, they often run segments of circuits to junction boxes and then a home-run from a junction box back to the panel where the circuits originate. When they do, they tie all the EGCs together in the junction boxes. They should take care to keep grounded conductors together with their associated circuits, but in my experience, they don’t. They tie all the grounded conductors together. You can be sure that if they don’t properly segregate the grounded conductors, they certainly aren’t going to segregate the EGCs. As I said before, though, they do not need to segregate the EGCs. I’m merely describing what I see on a regular basis.
Sometimes in attics, crawlspaces, and basements, conduit is used instead of NM cable for portions of circuit runs. Say, for example, a large ranch style house has the service or a subpanel a long distance from many of the circuits. The electrician might run a 3/4" EMT for a significant portion of the distance with several junction boxes along the run. That way, the electrician saves both time and money by running fewer conductors than there would be if all NM cables were used. Instead of, say, 10 or 12 home runs in NM, each with its own EGC, the EMT could be used as the EGC in lieu of the multiple EGCs in cables. Each junction box is going to have all the EGCs tied together and a pigtail to the junction box itself. That virtually eliminates any possibility of there being any isolated EGCs anywhere in the house.
In the city and county where my electrical contracting business was based, we had local codes that required EMT, IMC, or RMT in locations that the NEC does not. I also worked a lot in Chicago and they had similar rules. I’ve worked in other jurisdictions where the NEC permits the use of NM cable but local codes do not. The conduits further decreased the possibility of the existence of any isolated grounds.
To be clear, even if the circuits are all run in NM cable, the probability of the EGCs from any panel being isolated are infinitesimally small. I could see where a do-it-yourselfer might install a small two or four circuit panel in a detached garage or shed where they didn’t have a clue about what they were doing and end up with isolated EGCs, but I’ve never encountered such an installation. The do-it-yourselfers almost always, close to 100% of the time, tie the grounded conductors and grounds together in small remote sub-panels. In fact, that was a common practice for electricians until relatively recently.
So, while it is technically possible to have the grounds isolated, it just never happens in the real world.