Its the difference between using a thermal scope to spot a moving vehicle through its hot engine and using it to try to find a vehicle whose engine is not active and still cold. Also, the Reaper was deep underground and not active, whereas the ones over Mars were moving and putting off massive emissions that could be easily detected. It lets one masquerade as one to defeat the Omega-4 Relay. Reaper IFF does not allow one to detect Reapers.Also, there were geth jamming towers all over the area that could have made detecting it from orbit more difficult. got close enough to tag it with the targeting laser. The Rannoch Destroyer could have been in some kind of 'silent mode' to keep it from being detected until Shepard and co.Then why is it that no one is aware that a Reaper is present on Ranoch? There are a lot more questions about IFF, like how the Salarian are able to equip their dreadnought with IFF but human can't (considering it is on the Normandy), but I just like to know how Normady couldn't detect Reaper presence on Rannoch. The Normandy is equipped with IFF device, in the Mars mission it is shown that EDI is capable of 'detecting Reaper presence'.

So maybe the Reapers did send an advance patrol to the Andromeda galaxy, only for it to be defeated by one of these forces, after which they learned not to bother with Andromeda.

In Andromeda, we learn that the Jardaan were even more technologically advanced than the Protheans, and then there are the mysterious beings behind the Scourge, who were so powerful that they could weaponise dark energy and use it to distort the fabric of space in a large chunk of the galaxy.

They were created to solve a problem that happened in the Milky Way, therefore, that's what they do.

For all their intelligence, the Reapers are still bound to their own programming.

Most likely they just hibernate, unless Andromeda will say otherwise.

There's never been any indication that they go to other galaxies.

In that 50,000 cycle, between it, is it possible the Reapers 'hibernating' is actually them reaping other galaxies? Remember in Mass Effect, Sovereign is trying to allow the citadel to be used as a relay to get his buddies back from 'dark space', maybe they're actually in the next galaxy, with its own citadel style relay to get back to the milky way. Sovereign said to Shepard that basically time is not at all a problem to Reapers. At one point however the Codex mentions that some Reaper tech appears to violate known Physics. Other galaxies would take longer (Messier 33 would take 249 years). That order of magnitude is certainly not beyond the reach of Asari or Krogan life spans, let alone Reapers, and even Humans could manage colonization given minimal attention to cold sleep. Applying to Andromeda (Messier 31), it would take around 85,000 days or 232 years to reach there.

While galaxies would take a long time to get to, we know from the Codex that Reapers manage 30 light years per day (half again as fast as a dreadnought). It would be very stupid if they only did the Milky Way, as there are very likely many space faring species in other galaxies.

Mass effect reaper ships