Relationships are about trust and if you have to play detective in your relationship, then it's time to move on. Too many people hold on even when they do not trust their significant other, hoping to change that person. You can not change others you can only change yourself. If you are with someone and they are not truthful or they hide things then they do not truly value you or the relationship. Someone who lies, hides things, or is deceptive in other ways is immature. This is not someone that you want to continue with because their immaturity may show in other ways like cheating and playing mind games as well. A good relationship is one that is built on trust and honesty, where both people are equals and are treated with the respect that they deserve.
If you find yourself playing detective because someone that you care about is hiding things then your time could be better spent moving on instead. This is a sign that the relationship is not one that is good for you, and the other person does not have your best interests at heart. Someone who truly loves you will want you to be happy and feel secure in the relationship, and they will want to share their thoughts and experiences with you. If you can not trust your partner then the relationship is only great in your mind and you need to move on in order to protect yourself before things get worse. This person will never be loyal, and the relationship will never be as good as you hope as long as it is missing trust.