Thanks SirPleb.

I often control my survey ships so as to avoid wormholes and find the juicier planets to colonize, but do automate them later in the game.  Even when the entire map is visible and my surveyors have access to all of it, my anomalies are all gone, never to return.

I've tried avoiding cntrl-n to get a playable map, instead going through the main menu.  I don't think there's a time limit since my anomalies never seem to re-spawn.

Maybe there's a bug that steps on things with certain game settings.

At some point, I'll probably try re-installing the game.

Anomalies do respawn, but their rate doesn't always appear to be the same.  If you have sufficient survey ships running around, particularly if combined with a low anomaly setting, as SirPleb suggests may be the proximate cause, it's certainly possible you'd see large time periods without any anomalies.

Failing that, it may prove helpful to delete all files in your My Games\GC2 folders, to reset the game to its initial state.  Generally this clears up problems that the cause cannot be quite located for, but I'm unaware of what it would take for some change in the xml files therein to have this effect...

There is no respawn setting, however.  They either do (DA 17 or 1.8 and later, TA) or don't (DL, IIRC).

It seems that for anomalies to re-spawn, you must set your likelihood of random events to something besides none.  Although not definitive, with this set to none, I made it to the end of 2233 without re-spawnage and have been using this setting for quite some time now.  As Sole Soul alluded to, the rate may be relative to your likelihood of random events setting.