I don't care so much for identifying unique ships, as tracking ship classes. If a vessel with the Thermal signature of known freighter class and with an EM signature of 'no shields, no active sensors' comes wandering past, I'm perfectly happy with my picket ship classifying it as a 'Mule class freighter' and just letting it go. If it turns out to have been a hostile alien pretending to be a freighter, well, good for them. They earned that free passage and I have no problem giving it to them.