There is a utility website witch I presumed used a backend API. There is no authentication or anything on the website, it's just for public use. Now, I want to build a mobile app that makes use of the API. So I used good old Chrome debugger to see all XHR requests and responses going in and out. I figured out and listed down the API calls. Some use an API key (clearly visible in debugger captured request headers), and some don't need the key- they work with or without it.
There are no terms of usage on the site, or copy-write conditions, or anything of that sort, nothing. There is obviously no publicly documented API for the API that it is using either.
Whats my situation here with using the API even for personal use? The mobile app is not for commercial use. It's for my own use. Is what I'm doing legal?