A number of Zenfone 2 owners have been making oddly similar complaints about a lack of storage space, and it looks like this isn't a case of poor management on their part. There may be a bug in the device that clogs the internal storage with multiple gigabytes of log files. What's worse, you can't do anything about it without root.
All devices create log files for various reasons, but logs are supposed to be rotated (trimmed) so they don't take up too much space. It would appear this isn't happening on the Zenfone 2. What's worse, simply examining the folder structure won't tell you what's going on when the internal storage starts to dwindle because without root, apps that analyze storage can't drill down into the giant "system data" area.
Some users have taken to rooting the device to get a look inside that directory. They found that a sub-folder (data/logs/modemcrash) is getting massive in just a few days, occupying almost all the available storage. This folder can be deleted, but again, you need root. Asus has yet to respond to the reports, despite a multi-page thread on the official Asus support forums. The bug was apparently introduced in a recent OTA, so hopefully it won't be hard to fix whenever Asus gets around to admitting it's busted.