From my experience it's usually your mail app, your SMS app, or Facebook.
Do you send email from your device? Have you sent email from your device that had attachments?
If you answered yes to either of those or both try this: Go into your mail app and make sure you are not saving copies of your outbound items to the device. I'm not sure how to do this with all apps but with Maildroid you open each account individually and go to account preferences. There you can do two very important things, one you can make sure you are not saving outbound items to your device, instead direct them to be saved in your sent mail on the server. Two direct your deleted items to be moved to your trash folder on the server. Both of these will save you kind of unseen space on your device, because for whatever reason the space taken up by such things don't show in your memory usage. Not sure why.
Do you have a lot of MMS in or out bound?
Consider cleaning out your closet, so to speak. Not sure how this works on any other SMS app, but with Handcent you have an option to only keep messages for so many days or up to so many period. This option should automatically delete or archive your messages, not sure which or it may do both, and free up space in your memory. Again this is one that doesn't show in your memory usage, all of the time*. It may depend on your app or its set up.?
Do you Facebook, I mean FACEBOOK on your device?
If you are a moderate to heavy FB user from your device the app doesn't always handle the temp data the best. In the past I seen a lot of advice to go to the play store and get a third part FB app instead of the stock or Sense FB app, both of which came preinstalled on your device. The preinstalled apps don't seem to handle the deletion of temp files very well, and tend to clog up. I've read cases where the whole phone slows down due to the overload of these items. You can try from within the app to clear these items but the best solution is often the direct one, going to the app manager>All apps locate anything related to FB and deleting the cache for each one, and then restarting the device. The data may build up again over time meaning you may have to do this again at some point but at that time you'll know what to do. That is if it worked for you this time. You could also try a cache cleaner, but sometimes those come with their own set of headaches, ads, notifications, app killers (battery killers), and so on, but again I haven't tried everything so you may very well find one that works well.?
Speaking of App Killers, if you use app killers, battery managers, or memory managers these can sometimes cause issues as well. Killers are for android 2.1 and lower. Nothing running any up to date android OS needs one and you can see hundreds of posts and threads to that affect across the interweb. Battery managers after android 3.x are out too. Same for memory managers. These two usually end up costing the user more in battery life than the time needed to do any incidental touch ups, and are usually not reliable once you get into the upper 3.x and 4.x versions of android.
Good luck and please let us know if any of these worked or not. Thanks.
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