One of the features of a drop in liner is that the PVC liner is not bonded directly to the tank or substrate wall.  Besides the benefits of not having to sand blast, do extensive prep work, install the liner yourself, no cure time, etc., by not being bonded there is a natural space between the liner and the tank wall that allows for easy leak detection versus a bonded liner or applied coating systems.  There are several leak detection systems that are simple to install and very inexpensive to incorporate with your drop in liner.