The first thing I reccomend you to do is to somehow get proof that someone was in your yard, from animal control. Police will ask u specifically if u have proof. Whos to say yoir neighboor just thought it was someone from animal control, it might have been just a random person tresspassing. With stuff like this, where animal control is involved, they r going to have record of it. Sometimes they will notify u, sometimes not. Get a lock on that door stat. Ask your county animal control about them entering your back yard and get info from your neighbor about what exactly theyy saw. Best of luck