But, if you do them your way, you could end up searching for a reset anyway. I would argue the chances are about the same actually.
Let’s say you trip all the known GFCIs’ but you still have a bathroom receptacle energized. Now your choice is to write it up as not GFCI protected, or go searching for another controlling receptacle. If you write it up as not GFCI, you may get blowback for being wrong. So the best option is to go hunting!
The debate over using the test button or a 3-light tester goes on…and on. I do some of both. Generally, I start with the test button on the GFCI and try to see what is controlled by it. If there is a fridge in the garage, I always use the GFCI test button first. But heck, even then, they sometimes don’t reset.
Yes, GFCI manufacturers say that the test button is the only acceptable way to test their devices, BUT, it is common practice in the construction industry to use the 3-light tester button, so I will not apologize for using it. I have spent as much time walking around a house confirming what outlets I killed with the GFCI test button as I have trying to find the controlling GFCI. If I can’t find the controlling GFCI, then I write it up! And move on.
That is what I did also… 
No, because you only trip the GFCI with the built-in test button and then if receptacles downstream(in required locations) do not trip, them you write them up. Simply report that X location does not appear to be GFCI protected. If you want to risk trying to find out if it is on an unknown GFCI circuit then use your 3-light tester, just good luck finding that reset. We just choose to not do that and have not had any issues. But, hey if it ends up being covered by a GFCI and it does cause some grief, we just apologize and move on.
Hey Kevin, hope you are doing well.
At the other end of Ohio, the reset for the exterior GFCIs are almost always located in the garage or on a wall just inside the front or rear doors.
The hardest reset for me to find was a community bathroom one, which was located behind a stove, on the opposite side of the same wall as the GFCI that was tested.
It is awesome that Nick helped you! Way to go Nick!
Lon,
I had a similar situation with a seller and, like you, I “found a major stupid defect for my client who was not him.”
Here is a snapshot of my report on that DIY basement remodel
Hey Dave, All is well here in SW Ohio, but I wish we had a lake close by, a real lake, preferably full of Perch & Walleye. 
I don’t think I’ve ever seen a GFCI devise located inside the front or rear door unless the door entered into the kitchen or breakfast nook.
Here, depending on age, exterior receptacle GFCI’s usually re-set in the garage and if not the garage, basement, if not either of those, then bathroom.
I have inspected a couple houses that were built in the 90s that actually had the reset for the bathrooms located on the exterior of the hosue, I know, weird. That’s why I always include the re-set locations in my report.
The possible downside of being wrong is that someone installs a GFCI in that location and now we have two GFCIs’ on that circuit, which we all know is a PIA, lol.
If you are using NACHI SoP, you have to interpret this. NACHI should not be satisfied with this standard IMO. @gromicko now that you have been a party to this, would you like to weigh in? Maybe have a contest for authoring the best GFCI/AFCI standard for all of us. 
The high-end houses where they don’t want outlets in the kitchen backsplash always throw me (outlet strips usually on the underside of the cabinets). Ideally, they just put GFI breakers in the panel but a lot of times the sparkys hide the GFI resets in cabinets or under shelves in a pantry, etc. I’ve spent some pretty ridiculous amounts of time looking for them. Same with spa tub GFI resets. I’ve found more guns and porn stashes than I can count while rummaging around in people’s closets looking.
Hope you stop your “rummage” once detected and note “restrictions”… 
