How to find your first aliens?

[Saibot (OP)]

I know that you need to see them on active sensors in order for them to show up as a known race, and you use a transponder to let them know you exist, but more specifically how do I find the aliens to ping with my actives? Does the game interrupt when things show up on passive sensors, or only when a weapon is fired? What do I need to look for so I don't just blow past an alien ship or colony during a time interval and miss my chance for first contact?

[Rich.h]

Depending on how your event filters are set, you can get an interrupt for passive detection using either thermal or EM sensors. This can apply to all friendly contacts, all neutral contacts, or all hostile contacts, or any combination of those you wish.