In popular mythology, this is usually conflated with first contact, or at least with the establishment of genuine communications between the species in question – which portrayal, unfortunately, is almost pure nonsense.
Interstellar civilization just isn’t that subtle.
Space is cold and dark. Interstellar life is the exact opposite. Between the EM penumbra, starship drive flares, the gravity-wave ripples of stargates in operation, and even some few modified stellar spectra, anyone within a couple of thousand light-orbits of the Periphery with any astronomical competence at all can have no doubt that there’s exotic life out there – with the only possible exception being those on the wrong side of the Shadow Veil.
If you’re actually trying to make contact, you can’t avoid giving advance notice. In the first first contact on record, the galari identified Extropy Rising – a slowship, not even a lighthugger – light-months out of their system, even before the inbound ship spotted the radio emissions of galari civilization. The deceleration burn of a modern lighthugger is easily visible from the next star over, and highly distinctive to boot; an optimized fusion torch or the double-peaked signature of a pion drive look like nothing else in space. As for starwisps – how many stars do you think there are that shine monochromatic green?
(And if the lighthugger in question is a linelayer, it’s going to leave a stargate megastructure orbiting in their outer system for them to look at for months, maybe even years, before a scoutship gets there. Conveniently engraved with instructions for use, even.)
This does have its disadvantages, triggering social unrest, cultural shifts, bursts of technological development, and the like, or on less developed worlds – the kind whose occupants may go unnoticed until your arrival – sometimes even religious movements. In the case of psychotics-in-waiting like the skrandar, it may well have converted them into the berserkers they ended as.
But if you want to explore the galaxy at all – well, what can you do? Even the Voniensa Republic, who are remarkably prissy about this sort of thing, have had to reconcile themselves to that.
– A Star Traveler’s Dictionary