The silicone will dry out after awhile, but you can just reseal. My current shallow tank was in storage for about 6 years, silicone was shot. Resealing took a few hours and good as new.
Silicone doesn't 'dry out'. I've had tanks sit empty for close to a decade and had no problems with them as long as they weren't physically damaged. If the structural seal between the glass sides is compromised, no amount of 'resealing' will fix it.
Most tanks sitting in a garage shouldn’t have any issue. Just look over the seams and make sure they look good. Hopefully the tank didn’t have any heavy boxes on it and or mishandled.