Putting the gun under the seat is completely ignorant, well, In my opinion. Why would anyone want to do that? It should either be on your hip, or in a lock box. Ive only done it once, and that was simply because my lock box wasnt with me, and I didnt think to grab it on my way out. I had to work, but still, locked the car properly, in a security guard patrolled area.
Theres several problems to placing it under the seat, [passengers in the car] may be able to gain access to it without your knowledge, and, if your car has nothing to stop it, it may slide out and get lodged beneath your gas pedal or your break pedal. Not only that, but its an obstacle to get past, when in a self defense situation.
Theres almost no benefit to storing it there, Id hope you dont ever consider it.