If I scan and bookmark the location of a wormhole which ends up decaying, how likely is it that another wormhole or other anomaly could appear at that location? Are the possible locations of anomalies defined and finite to some reasonably countable number, or can the locations be completely random and I now have myself a nifty safespot bookmark?
We don't know for sure as CCP hasn't announced any specific information from what I can tell. However, our best guess is that sites appear to spawn within a 4 AU sphere around celestial objects. Eve University, a respected in-game corporation for new players with a wealth of information, makes this exact claim in their Scanning article of their extensive wiki. From what I can find, no better information yet exists to dispute this. Eve-U also has a note that anomalies may only appear around planets, but they make it clear they aren't very sure if that holds true or not (and a good way to disprove the planets-only theory is if a planet-less system has anomalies in it).
In practice, I've kept this in mind while exploring myself and it does seem to hold true. Since I'm playing as an Alpha clone, my scanning skills are limited and I've used this information to make educated guesses on where a site might be when my scanning window gives me a large potential area for a cosmic signature. So, this information appears to at least be mostly correct, and I've yet to really find a good exception to this that I could point to.