Can you share a drawing with the data?

I've found that manometers with data logging capabilities are super helpful when we know we have pfe, but we're still losing. It's really nice to know where and when you lose pressure (assuming that is what is happening).

Also, I've seen a situation where an ecosense products gave us bad data. Look at the data really carefully and make sure it makes sense.

This sounds like a fun one. As a building scientist and civil engineering technologist it was always drilled into me to treat the building as a whole until you figure out what is going on. May I suggest that approach here as this may go well beyond simple PFE.

What is the HVAC doing? Since it is an MURB the HVAC will be more complex and even more so if each unit has control over air flow in any way (e.g. call for heating or cooling). If you can use some Corentium Pro CRMs you can really manipulate the data and graphs which may offer some clues.

What type and configuration is the PT slab(s)? You give a good description, but the devil may be in some small detail or construction defect not yet seem. Do you have access to the engineering drawings.

Were the filled sink holes mentioned within the footprint of the building? How were they filled?

My last question before Bob Wood beats me to it LOL. Is the rock system with the caves that the site sits on exposed to a cliff face anywhere in the near vicinity. Could wind pressure be messing with you in a strange way?

An amateur here with some thoughts in the hope that one or two might be helpful:
Agreeing with Bruce's maxim: "Take a whole-building approach until you show it's not." This tack suggests that there might be other pathways than around the slab or through joints/cracks, etc. Hidden utility chases, who knows?
Re Shawn's: "Can you share a drawing with the data?" Yes, the experts (not me) on this list can give you a lot better advice if they can also see all the details you have.
Re Doug's comment: "All areas checked have shown neg. pressure below the slab." Are there any areas you haven't tested yet that could reasonably explain those high radon levels? Are there any diurnal/meteorological reasons to suspect that your neg. pressure values do not remain stable? And perhaps you don't need to go to the trouble of making test holes everywhere if you would first use IR vision to look for any obvious places of incursion of soil gas?
Re Bruce's comment: "Is the rock system with the caves that the site sits on exposed to a cliff face anywhere in the near vicinity. Could wind pressure be messing with you in a strange way?" Makes me think that shared variability in the behavior of multiple simultaneously run CRMs, tracked along with met. data, could indicate some pros and cons for that hypothesis.
Re Doug's comment: "...before drilling and some testing of the concrete for emanation." I'll take this to mean that you'd be hypothesizing that the concrete itself is the source. Unless you have experience with that happening in your locale, or if this site is a former radioactive tailings dump, I'm inclined to think that radon levels you found are too variable / too high at the high end to think this explanation is likely. I'd save the emanation question for later unless there's an easy-to-do standardized procedure for determining if it's significant (Bill Brodhead, what do you think?).

Are there sub slab or even sidewall penetrations that are below grade? Below grade HVAC, Electrical etc. can move soil gas a long way in unexpected ways.

i have to chime in on this one,
#1 i think the idea of checking concrete is a good one simple to do and it eliminates a source if it is negative.
#2 You still have pathways into building if you are having high levels of radon and it is not emanation
smoke test may locate those openings fairly easily big smoke machine may be required.
#3 What is subslab pressure, is this normal for your area? is it bouncing a 24 hr 7day charting may tell you something significant.
#4 Do you have block walls that can be chimneying radon into space i have only had to do this once but when we were stumped, we grab sampled the block and it was super hot. When we tapped block with a branch from each of 4 suction points near the walls (we had 10 suction points by then) it solved the problem.
good luck!

Thanks for all the great input! The solution turned out to be “back to the basics” seal seal and check the seal job again. The slabs have a 1 sq. foot open area under each tub drain that is open to the gravel base. Each under tub area needed to be accessed and the area sealed well. 8x8 inch openings and access doors are cut and installed. We use cans of spray foam with 36 inch straws attached to get to the slab openings. The foam tends to bridge and leave openings which needed to be re-sealed following initial cure. There are 12 to 16 openings per space…so an equivalent large opening when unsealed. Again thanks for the input!

There will be approx. 100+ systems when completed and the client is thinking they may want a remote monitoring system that sends an email or text alert when the static pressure is out of spec…
Any ideas?

Thx….DT

Congratulations on your detective work and on being rewarded for your application of first principles! (Glad it was none of the wild scenarios I and some of us were thinking of.) Those large but hidden breaches through the slab under tubs and showers keep showing up in "houses from heck," so this is a good reminder for mitigators not to forget to explore the existence of those, if not too much trouble, even before doing the first round of post-sealing PFE testing. For example, fixing those first perhaps could reduce the need for a more powerful fan even if a system might have "worked OK" without addressing those gaps.

And yes, there are such telemetry devices as you're asking about on the market, and I anticipate that these will become more common, as folks become more and more accustomed to the "Internet of Things." I leave it to the experts to discuss their costs, capabilities and limitations.

Talk to Gunnar or Matt at Obar Systems 1 800 949 6227 has some great low cost systems that we use for commercial projects, they are really knowlegable and helpful. tell Bob sent you.

The plumber may only need about 1 1/2" for the trap under each tub on commercial sites IE multi story they now do not creaet a box out to gravel they insert a 2" piece of blue foam for tub outs at finished level of the floor.