Someone is stealing my gas?

After vacationing for a week, my car has gone without use. I drove it today and noticed that there was less gas than before when I left it. I also noticed that the gas tank cover of another vehicle that belongs to one of my family members was open. This has happened before in the past and seems to be resurfacing at about the same time of year. If I'm able to catch the person doing this, am I allowed to restrain them or deter them with tools that are at my disposal? Or will I be getting a visit from state police for unlawful restraint? This is burglary and vandalism of my property and I will not tolerate it.