Do Japanese destroyers always ping?
 

I remember once at Silent Hunter III, in a mission where I do have to sink one of three battleships, I looked at the situation when I spotted a destroyer sailing too close to me, I feared detection so I lowered my periscope and rigged for silent running while following him with my hydrophone... Using external view I saw that I did completely passed right below him, but I was asking why didn't he ping me and detect me? The answer was pretty clear: I didn't make them suspicious about the existence of a submarine there, no loud noises, no periscope above the water, no torpedo waves nothing

By then when I passed the destroyer screen I fired a salvo of four fish towards the second battleship in the line, and I sailed to the opposite direction they were sailing, and I just watched them circling and depth charging the "nothing" that they though it was a u boat

When the whole task force passed, the battleship that I hit took a ~5 degrees to starboard and lost all power, I fired a salvo of three and sunk it...

If the first destroyer pinged me, I would have been detected so battleships would be zigzagging, and no one of them will be sunk

Is it the same in Silent Hunter IV: Wolves of the Pacific? Can I pass below a destroyer without him pinging me if I don't make them suspicious?

Yes - that's fine.
However, you must remember that the Pacific is much clearer than the Atlantic Ocean ... you may still see you at a depth of 30-40 meters. :03:

http://www2.pic-upload.de/thumb/3255....15.13_980.jpg

:salute:

I can't answer that question for real life. In game, it depends on the skill level of the AI. For stock or vanilla game, it's random. Mods will change the skill level. If the enemy AI crew is at the lowest level, you'll get the situation you just described. If the enemy AI is at the highest level, then they ping constantly and can detect you if you are running silent below the thermal layer, on a dark and stormy night.

They do spend their entire life - flashing lights (messages). I have extensive experience of convoy insertions and they only seem to ping on detection. Mind you, all conditions, mods, approaches, speeds differ.