Harddisc space magically disappearing/reappearing

Been noticing a lot of strange behaviour lately. My Harddisc space is seemingly fluctuating by a great deal.

I went away to my cottage for the week and brought along my MBP for offloading photos and the like. Before I left my hard-disc space was around 200 gb. It was seemingly lower than I thought it shoudl be, but didn't think much of it.

A few days later I pulled up the machine at the cottage to offload some photos and noticed I suddenly had a whopping 228 gb. I hadn't done anything - no deleting, nothing. It was incredibly bizarre.

While at the cottage I loaded on, maybe 500 mb of photos, as well as a few movies to watch on the rainy days which I subsequently deleted once we were done.

Got home yesterday and went to check and noticed I was down to something like 187 gb. What the heck? I hadn't downloaded anything to take up *that* much space, and what little stuff I was downloading was going onto external harddives anyways!

Since waking up this morning I've noticed my space fluctuate from 199 to 198.6 -- all I've done is browse the web update my phone.

I read somewhere that Time Machine now does local backups so that could potentially by it I'm not sure. What I don't understand though is how I could have such a HUGE fluctuation. A gigabyte here and there I can understand but how the heck did I gain nearly 30 gigs over the period of a few days only to lose it again when I came home. The only major thing that changed in that time was while I was away I was *not* doing time machine backups, and then when I got home plugged my TM drive back in and resumed backing up. But if anything I'd think the local backups while I was away would use up MORE space, not result in my GAINING 30 gigs.

I know this is all rather meaningless since ~200gb is more than enough internal space but I'm anal about how much space I have on my machine. But more importantly if there's 30 gigs just floating around I'd like to have them back. I cannot for the life of me figure out what is going on.

Any help would be greatly appreciated.