Since I never gave consent for the police or my girlfriend to take possession of my lock box, could the it be an illegal seizure?

My question falls under both criminal and civil law, but my main concern is mainly dealing with the civil side of the issue. I am the target of a grand jury investigation for criminal damage to property over $1000 and attempting to commit a felony (breaking and entering). These charges are the result of an argument with my girlfriend. During this argument, she took property, a lock box, that belonged to me and I attempted to retrieve it from her vehicle without her consent. My girlfriend admitted to the police that she took the lock box from me to the police and they in return told her to hold on to it. She also told the officers that she thought that I may have a fire arm inside the lock box, but she had never actually seen inside the lock box and was not sure. She asked them if they were going to need it for their investigation. They told her that they would not. Approximately three weeks later, the officers contacted her saying that they wanted the lock box for evidence, and she turned it over to them. I never gave consent for my girlfriend or the officers to take possession of my property. The officers are also stating that they are going to attempt to get a warrant to make me open the lock box. The reason that they want the lock box and want to look inside it is because of the statement my girlfriend made about the possibility of a firearm being there and the district attorney wants to try to charge me with felon in possession of a firearm. As far as I am aware, the statement that my girlfriend made about the firearm is hearsay and would not be enough for them to obtain a warrant. I also wanted to make it clear that there is no firearm in the lock box.