I have a 256GB SSD in my 5,5 MacBook Pro running OSX 10.10.1.
It's mostly filled up, mostly with music. When I get close enough to "full" I usually delete the less-important stuff to free up a good 5-10GB, then slowly that free space disappears, even though I haven't downloaded anything (besides streaming Netflix, browsing the internet, etc).
What's going on here?
Sadly for you, lots goes on in storage.
Apple has several caching and thinning features that will proactively reclaim space when you run low. Unfortunately, you'll likely need to maintain more like 15 GB of free space if you wish to surf on the wave Apple has set for you.
To solve this, you'll need to be specific about how you are measuring free space and track that over time. You'll get different answers about "how much space" is free depending on which tool does the measuring.
So, if you can afford to relocate 30 GB of files temporarily, do that and see once things stabilize and Apple's cleaned things up. Then you can measure and rejigger things.
If you can't afford that - then you'll need to dive into things and not count on the "automatic transmission" to shift gears. I use Daisy Disk (and What Size in the past) as well as terminal and finder to help in cases like yours.
If you go down that manual management route, see:
