OK, I've checked our logs. All access to your web app has been from the same IP, so I think it's probably not someone else using your account (unless you were away from your machine and someone else used it). I can see a number of "reload" requests for the app between around 9:20am and 10:33am, then a request to delete it at 16:10. You then re-created it at 16:29. All of the requests came from what looks like the same browser, and definitely the same IP.
The only way I'm aware of that that delete request could be made is by someone clicking the "Delete" button at the top of the "Web" tab, but presumably you would have remembered doing that... There's a warning dialog, anyway, so I'm sure if you had clicked it you wouldn't have been surprised when the app disappeared.
Conceivably your browser could have made the delete request without you knowing about it... do you have some kind of plugin that "pre-fetches" URLs it finds on a page just in case you might follow them later?