Goaroundflap20
There are a couple of different perspectives to view the scenario you mention.
With a triple redundant system, I would hazard a guess that when you look at your emergency cards at the subsequent actions, there is no mention of 'Land Immediately', 'Land as soon as possible', or 'Land as soon as practicable' (or whatever the terminology is now... I'm referring to military flying days). Therefore there is no need for you, as a pilot to call a PAN... as has been stated, it is a bit of a non operational event.
However, ATC need to know for the purposes of possible runway contamination, therefore as a pilot you need to get the message across that you have had a hydraulic failure; you can continue to operate as a normal flight, and that it is for info only.
Unfortunately in these days of heightened security etc, visits by ATCOs to the flight deck (certainly in the UK) are hard to come by thus leading to less exchange of knowledge as to how the person on the other side of the microphone operates, or what he/she expect from either ATC or from the pilot.