Hi Tomas Coufal,
Try opening the remote Thing entity and the check the change history details for which user created this entity. You might get some hint from there.
I hope it helps.
Tomas, it is the websocket session that gets created from checking the 'Automatically update when able' setting on your Mashup. It essentially creates and maintains a (websocket) connection as long as you are accessing that Mashup.
If you login through an incognito window, for example, and view the same Mashup, you are pretty much creating a 'new' session, so another PersistentSession Thing will pop up.