I completely agree with Mikkel. 'Find' does not only refer to encountering objects - you can also 'find' that something is the case, as in 'I found that I slept better if I didn't drink coffee'. There's nothing incorrect about the first sentence. I think it's perfectly normal to say 'I found the thief had broken in.' It simply means that I saw or realised this. In fact, I'd even go as far as to say that it would be more natural to say 'found' rather than 'found out' in this context. 'Find', when followed by a clause, usually refers to a simple realisation of a fact. For example, if you get home and find your window smashed and your belongings missing, you might very well say 'I found the thief had broken in'. 'Found out', by contrast, often involves a third party - either a written source or another person. If you say 'I found out the thief had broken in', this suggests that somebody told you that this had happened.

I have to disagree with the others that “I found the thief had broken in.” is incorrect. Perhaps it’s more literary language and not so common, but I don’t see why it would be incorrect. https://en.oxforddictionaries.com/definition/find : Definition 1 of “find”: Discover or perceive by chance or unexpectedly. Examples: ‘Lindsey looked up to find Niall watching her’ ‘As I walked into the room I looked around to find Mark watching me intently.’ And here’s an example from the novel Till Then My Love Forever: “I found the thief had stolen a couple of things off the kitchen table, but that’s as far as he got.”

As Steve mentioned, the first sentence is incorrect. Find- Is used when you were looking for something and found it Fount out- to become aware of a fact, learn something you didn't know before

The former is incorrect though can be slightly modified to make it correct. "I found the thief"... I searched for and met the thief. "I found the thief, he was hiding in my garden" "I found the thief who had broken in", "I found the thief who had broken in, he was hiding in my garden" Examples: If a child plays the game of hide and seek, their playmate will find them. If you work on a maths problem you find out the answer. You can find money in a wallet. When you find the money you can find out how much was in there. If you break a bottle, someone may find the broken pieces and if they ask someone who saw you break it then they will find out it was you. I found out his father was dead. They found him in the forest. Find means meet/encounter Found out means (often) new information comes to you Edit: Sara put it better below. To find out is to become aware of something.