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This is entirely based on Are Alien "Lurkers" Watching You? by Cool Worlds.
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Viable options and limitations for detailed visual observation (of us):
| Location | Max size | Max surface resolution | Notes |
|---|---|---|---|
| Exo system | >star | <1m? | Solar system size to get useful resolution. |
| LEO | 10cm | 2m | Orbital decay, increasing detection capability. |
| Lunar surface | 100m | 3m | Increasing detection capability, dust hazard. |
| Lunar subsurface | moon | <1m? | Limited observation spectra. |
| Co-orbital asteroids | ~km-Mm | 4m | Limited observation time (close approach), unstable orbit. |
| L4/L5 satellites | ~km-Mm | 4m? | Increasing detection capability. |
| Distant solar orbit | planet | <1m? | Increasing detection capability. |
| Flyby probe | <1dam? | <1m? | Need to send continuous stream of probes for data acquisition, which leads to easier detection of such probes. |
Non-visual observations may use radio, but then there is no reason to send a probe near us, so we would never detect it. Additionally, as we grow more advanced, we tend towards leaving radio behind for better data throughput in tight-beam transmissions.
ʻOumuamua is not likely a flyby probe, it would only have surface resolution of Earth of 20m at maximum, and was tumbling through our system, which would make observation very difficult.