TPMS does not measure tire pressures.
 

S5, and probably A5 as well.
It uses the ABS sensors to measure wheel speeds. If one has low presssure, it will turn faster, and that sets off a TPMS warning. If they all bleed down the same amount over time, you will not get a warning.
Mine were at 30 PSI, should be 42. (Months since last check & fill). TPMS never noticed because they were all at the same pressure.

Check pressures monthly or more often.

That is an interesting observation. I don't know how the TPMS is programmed by Audi, but if it was only looking for a single (or more?) tire(s) to be at different relative speeds, then I'm not sure why there would be a 'set TPMS' (baseline) procedure. Do you know that your TPMS was set with all 4 tires at 42 PSI?

You sure they should be 42 PSI? That seems too high and is likely the max PSI. Please check your owners manual.

This topic has been discussed many times around here and your post is highly inaccurate. What you are describing is how the very very first generation of these indirect TPMS systems used to work and their flaws. Today's system haven't worked like this in a long time. Today's systems measure oscillations in the wheel assembly that are highly influenced by tire pressure and then through digital signal processing and spectrum analysis can detect if one or more tires are low. It does indeed detect if all 4 tires are low at the same time. However, TPMS only meets the law, so it only alerts if the pressure is 20-25% lower than the reference pressure that was used to baseline the system. Every time the pressure is adjusted or a wheel is changed etc. the system needs to be reset to establish a proper baseline. Failure to do so may result in false positives or failure to alert. My guess is your system was last reset at around 36 PSI, so 30 PSI is 20% low and not quite enough to trip the system.

Too bad that it doesn't relay the actual readings like my current car. I got a warning this morning. But knowing that 174kpa triggered it (lower than 182), it is no big deal and I don't have to check it.... now in the rain.

TPMS warning for a tire being 4.5% low? sounds like a nuisance (equivalent to warning for 34.5 PSI for 36PSI desired)

Sounds like operator error to me. The indirect TPMS should detect all four tire being low. My guess is that your set point is set incorrectly. Pump up all four tires to proper setting and reset the TPMS in your MMI.

Just a few more comments. A lot of folks seem to think that TPMS was designed to let them know when they have to put air in the tires. This couldn't be further from reality. TPMS' primary purpose is to act as a last resort and alert the driver of a significant loss of pressure in one or more tires. It does not preclude one from regularly checking their tire pressure. As said, these systems alert at around a 20-25% loss of pressure if operated correctly, and that's primarily meant to detect an unusual loss of air due to a puncture etc. Unfortunately, many car owners neglect this basic maintenance requirement and that's what persuaded officials to make TPMS mandatory in the USA. Some manufactures still use a direct system with sensors, but that incurs extra cost to the customer. For example a second set of sensors is needed for a winter wheel setup or the complete sensors have to be replaced if the battery dies. This extra cost is the primary reason Audi and others have gone with the indirect system. They have proven to be accurate enough to satisfy the law. Mainstream models will more and more go into this direction. Direct TPMS is still something that can be found on higher performance cars. In those cars it typically not only shows the current pressure, but also the tire temperature and the purpose of those is so that the driver can keep an eye on the hot tire pressure and temperature while busting around a racetrack for example and make necessary adjustments to achieve even pressure and temperatures. The RS models starting with MY19 will have direct TPMS standard for example. The gen 2 R8 had it standard already. For everybody else, OP is correct in one point, check your tire pressure regularly and visually inspect your tires. It's the only thing that keeps you on the road.

I agree with the ON/OFF signals like in the Audi. My realitty is that I used the actual readings to monitor if I needed to add air and it worked A1 in the last 36 months.

Not to nitpick but the Firestone / Ford truck lawsuit disaster in the 80's (I think it was the early 80's but I could be wrong) was what made manufacturers start to implement TPMS. That settlement cost Firestone a cargo ship full of cash and possibly bankrupted them.