The fact that SETI researchers examing "Tabby's Star" have found nothing, actually does mean nothing, for a number of reasons.
1. Most EM/RF traffic produced by our own civilization is indistinguishable from background noise after just a few dozen lightyears.
https://briankoberlein.com/2015/02/19/e-t-phone-home/
2. Sample size of one, but the only technological civilization we know of, us, is getting increasingly less noisy in EM/RF over time. Fiber optic communications, for example, generate no EM/RF interference and can't be electronically intercepted without physically interdicting the line.
http://www.ciscopress.com/articles/article.asp?p=170740&seqNum=6
A more advanced civ, even one conducting massive construction project in their own system, isn't likely to be very loud in EM/RF. If, for example, they were communicating with unidirectional laser packets you would have to be extremely lucky and relatively close to intercept such a message.
3. Maybe they just communicate in ways we simply can't intercept at such a distance. For example, maybe they communicate through complex pheromones messaging and send physical pheromone packets between ships/stations in order to communicate... perhaps electronic transmission and artificial reproduction of the pheromones just doesn't cut it for some reason. We would never intercept those kinds of communications without being in system.
4. Coming back around to item 1.: A lot of people way smarter than me have figured out that about the only way were are intercepting messages from alien civs is if they are either relatively close (20 or so l.y. for us to detect their leaking E.M.) or if they detected likely life bearing worlds in their own telescopes and deliberately send us a focused message.
5. Finally we'd have to be capable and willing to listen at the precise moment any such communications are sent our way, purposefully or otherwise. We've really only been capable of intercepting such a message for about 60 years or so. Who knows, maybe they sent us something 100 years ago and we just weren't listening. The next scheduled transmission might not be for another thousand years depending on their timeframes.
But really, it is very likely not an advanced alien civ in this case. Probably something we already can think of happening in a way we're unfamiliar with, or something new, but natural, we haven't thought of yet. Given how often planet hunters end up saying "Huh! Never would have thought that would be possible...." with even a very limited number of planets found so far, I'm sure we've got a lot more surprises on the way.
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