OSX update cache: does it accumulate over time?

Question

I bought my MBP 2.5 years ago, it came with a 240 GB drive. I don't remember how much free space it hat to start with, but let's just say 200 GB.

I mostly use it for personal use (different machine at work). I don't have movies/music stored on it (all on external drive) and have some applications installed (total app folder size: 20 GB). My ~ dir is also approx 20 GB.

My drive now says 140 GB free. I can't explain the 20 GB difference. I think apple isn't removing downloaded updates after installing. Where should I look?

Another idea was mail cache folder. Where is this folder?