Using 'can' and 'must'

I have took the tests recently for check my english skills and one test confussed me. There was sentence : I can't find my keys anywhere - I ...... have left them at work. I choose can, but it is wrong answer, the right answer is must. I don't understand why. Can anybody help me with that test, please? Thank you!

*to check my English skills, and one question confused me. 

The reason that the correct answer is “must” is that there would be no other option. The word “must” indicates this fact, as it is quite definite, because the person had already looked everywhere else, without finding the keys.