I would guess that the ‘battery usage by app’ list might provide a clue, but you mentioned that it did not. Is there a chance that iOS performed updates in the background? It could be some bad programming within an app that caused this. Keeping everything updated and offloading rarely used apps might prevent a reoccurrence.
edit: that currency app is at the top of the list on both screenshots, you could remove that for a while and see if the situation improves.