You’re not neutral to them, as evidenced by them obliterating you and your ship.
Don’t assume that no standing is the same as a ‘true neutral’ standing where they don’t care about you at all. By default they kill you, unless you gain positive standing with them.
Currently they can appear in any System that has wormhole connections.
If a wormhole to Triglavian Space (a 27 System wide area under their control named Pochven) opens up in a System the Trigs will come through, gate camp, station camp and generally be very annoying. This can happen by surprise with no warning and no intel reports in advance. You can literally jump into a ‘perfectly safe’ High Sec System that didn’t have Trigs in it earlier and find 20 of them waiting for you on the other side of the gate.
They usually hang around until the wormhole closes again, or until destroyed by players.
This is now just an unfortunate hazard of High Sec life. Sometimes you can see them if they are on your exit gate, or see dozens of wrecks, or hear the screams of dead pilots in Local chat. Watch out for these things as you fly and you may just avoid them.