It’s smart to close all the seacocks anytime you leave the boat. If it’s original, you’ll probably find that it’s a gate valve, and if it hasn’t been operated regularly, it may be frozen in the open position. Either way, it should be replaced with a ball valve. The anti-siphon loop is a good idea too for when you are heeling and the valve is open, also as backup for when you forget to close the seacock. As you discovered, you don’t want to depend only on the wet/dry lever on the head.
As for the location, mine is also on the port side under the v-berth, but I also have an H-30, so I may not be of much help there. Just follow the hose and it’s probably not far from the head. If you find the thru-hull is somewhere not easily reached, you may need to relocate it to where the seacock can be conveniently used as intended.