Keys to your Castle!
How many people do you know that have the same keys to their home that they had 20 years ago? Or even better kept the keys that the prior owner used. Did you even think that someone may have the key to your home. That they can just walk in anytime they want - go though your belonging - borrow things.
If you have not checked your locks it is time.
Had a contractor at my home some time back and he forgot the key I loaned him. He used a piece of plastic to get in. He then suggested new locks be installed - which they were.