Imagine this scenario: your partner is coming from work later than usual, they suddenly started not telling you where they have been or maybe they said that where they have been (or with whom) is none of your business. Sometimes they want to stay out with their friends later than usual and you're asking yourself what on earth is happening - the truth is that there is something going on that you're not aware of. Be ready for the day when you find out.
This is a very common sign before the end of a relationship. The person with whom you were together for years is all of a sudden acting strangely and they go out and do things that does not include you. It shows that your partner no longer has any interest in you and they want to spend time with other people which is why they have become secretive about their dealings. There is a very high probability that they are cheating on you - so, don't be surprised if you hear that from another person.