So what? It's unmanned. Worst case it sinks and the shipment is lost. By default it surfaces and waits for someone to come fix it.
Worst case it surfaces without power and another vessel runs into it. Hitting a lost shipping container is bad enough. Hitting a submarine is likely to be worse since they're structurally much stronger.
Getting a tow line onto that thing is going to be hazardous in any kind of rough weather. As often happens in the Irish Sea.
Inflating a bright-colored radio-reflecting balloon and triggering a radio beacon is really easy.
Does that make it any different from the millions of pieces of flotsam/jetsam all over the ocean?