It is NICE that defines what is detectable or undetectable, because NICE dictates what funding can be spent on salvage treatment and at which point. NICE says that anything below 0.1 is undetectable and therefore does not need further treatment. It also defines biochemical recurrence (after RP) as PSA higher than 0.2 or 3 successive rises above 0.1. Different thresholds for adjuvant treatment ... first post-op PSA of more than 0.2 and / or positive margins being 2 aspects.
Either way, you know where this is going and it all comes down to personal preference, doesn't it. John knew that his path results were poor, the uro gave him a 55% chance of needing further treatment, his pSA climbed upwards (but at tiny numbers) and he chose to a) pretend it wasn't happening and b) wait until there was irrefutable proof that the cancer was back (he held on until a PSA of 0.18 I think). Partly a good decision in hindsight; it gave him time to get his head round 'it hasn't worked' and recover as much as possible from the RP side effects before embarking on new treatment with new risks. Partly a bad thing because recent research shows that the outcomes from adjuvant RT are better than for salvage RT. Hey ho.
As for challenging her - it isn't easy to remember all the things that you think you would say in an appointment. I would be more interested in asking her whether she is aware that many hospitals have stopped offering the ultra-sensitive test because its reliability has come under fire. But my guess is that she is more interested in the pathology and her intuition is that your PSA is going to do the three rises over 0.1 at some point.