We get these calls every summer “my sink is backing up and it wasn’t even being used!!!”
If you are like me I don’t get upstairs much. I rely on my kids to let me know if they are 
having any issues with anything in our home upstairs. If your kids are anything like mine these notifications may not come early enough to prevent damage. Here is an example my daughter let all of the lights in her bathroom burn out. She moved a lamp into the bathroom instead of changing the light bulbs….. I was proud of her for being resourceful, however we needed a lesson in changing light bulbs.
The same goes for sinks. If you don’t check your sinks upstairs, it could be a huge mistake. Air conditioning units tie their condensate drain into a lavatory sink drain line in homes. In two story homes it is normally a sink on the second floor. The pipes in your home are warm if not super hot during the summer. The cold water that comes into these lines begin to fester and build up sludge. This sludge is what causes the drain to become clogged.
Most of the calls we get with sinks not draining in the summer are AC stoppages. When we get these calls it is normally too late and water is pouring through the ceiling or overflowing onto the bathroom floor.
Don’t delay, check to see if your bathroom sinks are draining slow.
Call Bear 281-350-BEAR (2327)
You can check out some of my previous posts on this topic
https://theplumbersbride.com/2013/07/17/sludge/
https://theplumbersbride.com/2012/03/16/ac-drain/
https://theplumbersbride.com/2015/05/11/is-it-a-sink-issue-or-an-ac-issue/
theplumbersbride 
In a situation like this do we call a plumber or do we call ac company? Or is this a quick fix on our own?
Some customers call both the AC company and a plumber. In some cases that AC company can push the blockage further down but not all the way out of the line. A plumber can unstop from the AC pan and the lavatory drain that usually is the cause for this type of stoppage.
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Thank you for writing this up. I had already tried a 25’ auger 2x, plunging, all from the horizontal branch line under the sink, to no avail. While I still don’t understand how the toilet & tub drain fine (I’m presuming they join up to a common drain line in the wall), this gives me a little more insight to why the sink is backing up when not used. 1 more auger attempt, then it’s up to the professionals. Thank you for all you’ve shared.
I am so glad you found this helpful. I also hope you were able to get your stoppage resolved.
Yes it was resolved. Now to just keep it maintained (and a teen daughter from adding more clogging factors to the equation..) . Thank you again!