Torps are not autonomously seeking. They are manually laser guided.
In fact, I mean based on the decay couldn't you just set a bunch of warheads to proximity trigger and dump them in an area? We know that arming a warhead can be done separately from other performance factors. In theory you could just develop a mine layer by giving it an extremely low velocity, setting the warhead to proximity and firing it right? Obviously there's some form of launcher based safety available on the proximity warheads that prevents them from assessing their own launcher as a valid object and exploding on launch. If the lifetime of a missile is based upon its fuel consumption and its barely consuming any fuel these could last for minutes at a time, right?
Those munitions wouldn't be lasting as I assume they've eventually decay or simply run out of fuel and explode in the case of slow moving ones, but as a short term or defensive solution it seems pretty trivial to implement. Just need a ship with a bunch of missile launchers at rear angles. You could load the launchers as a one-off button, or build a robot arm auto-reload system.
In theory this would allow a ship built for this purpose to turn on "minelayer mode" and just move foreward on a single vector and litter the space around it with these short duration proximity mines.
Commute this system to an automatically triggered YOLOL netowork, where if one of your minelaying devices detects a target, it sends a signal to the others to also trigger and you could have a pretty interesting on-demand and reasonably reusable minefield using a slow automated "chair sitter" drone to fly around and restock the missiles, or just by manually restocking the missiles.