Well lets assume there's probes somewhat equally distributed in the galaxy, and that there's many tiny probes and fewer larger probes. Assuming enough technolo

Well lets assume there's probes somewhat equally distributed in the galaxy, and that there's many tiny probes and fewer larger probes. Assuming enough technology for replicating probes the number of probes in the galaxy is limited only by time and materials. Presumably if you are watching a galaxy you are quite patient and a few 100,000 years is no big deal. With that kind of experience you've likely figured out what signature events are related to the rise of intelligence. It's hard to speculate on the technology, sensors, and intelligence of the probe makers. Maybe the signature even to watch for is plankton, oxygen, and being in the Goldilocks zone (billions of years ago)? Changes in atmospheric chemistry related to clearing forests and cooking? (1000-10,000 years ago)? Emissions of radio waves (75 years)? Whatever that threshold was, it might have been triggered in the past and a slow moving smaller ship was first (Oumuamua), now a few years later a somewhat faster (15%) larger ship is coming for a look. Fun to think about anyways... at least we saw this one coming so anyone that wants a closer look will have the chance.