I was looking through some of the Zurg images and realised there was something slightly peculiar with the one regarding the Stargate Warp Portal (WP) - have a look:

Now, on the left we have the Zurg Homeworld - one shaded Grey box which I am assuming is showing the location of their WP; on the right is Gaia - with TWO shaded boxes.
Now - on the Gaia maps, the WP is located next to the Great Wall of Gambino in the fairly lower half of the available maps:

So, going by this logic - there is a SECOND WP (like the second Stargate found in the ice).
By the location, if we compare the two maps roughly, the 2nd portal should be located somewhere just above the coastline where X / Gino-Gambino / Ian had their little fight and attracted the Zurg in the first place.

Now maybe, just maybe it wasn't the white light that attracted the Zurg; but what if it was the influx of power, perhaps enough to re-open the hidden warp portal?
With Gambino and Mother being friends to start, intel regarding the 1st portal may have been shared - but it's highly unlikely that if there is a 2nd that they would have revealed it.
On the 13th December, Gambino was interviewed by Cindy and said a few things that support my idea that the Santa-Cow incident was not the only thing the Zurg had in mind.

Here Gambino says "plans" as in plural - there's more than one plan afoot - it could be the Santa Cow idea AND the Zurg Drink ploy,

(this here shows "plan" as in singular, so we know it was only ONE of the ideas) but the next one suggests more...

Gambino actually comes out and SAYS that Mother wants the Zurg to rule the Gaians.
So - although they've "left in peace" it's highly unlikely it's going to stay that way - the Zurg have discovered that the Gaian race is advanced in some way they are not - and that means they're gonna want that tech for themselves.
(We know they have used the tech because of Flarn's resurrection and the strong possibility that a version the G-Virus was used)

(Notice the lab-wires etc - the only place where we KNOW this tech would be accesable would be the Labs at Gambino's mansion - that's not to say it's somewhere else; after all it's documented that "there is more to the feud than business competition" between the Gambino's and the Von Helsons.
(At this point it's also worth noticing that Cindy's report on April 05's event states "apparently killing" in conjunction with the Sisters - it has NOT been confirmed.)
Not to mention the curved walls look as if it COULD be a Zurg Space-ship; but the Gaian tech would still needed to have come from Gambino and the Labtechs.
Given all of this - it's almost assured that the Zurg will be back, and whether it's an attack from the sky, or sneaking in from another portal - it is only a matter of time.