Welcome to the Forum.
If you can’t find the/a valve and the lines are exposed, why not just cut them at the lowest point, then install T fittings with drain valves ? I would use pex T’s and plastic Pex valves. Then you have easy access to the drains... Here’s a few pictures of what I’m referring too. The picture shows a shark bite T but you get the idea. You can find everything at Home Depot.