Can someone tell me what this is?

Mine looks bit different.

 

Radiator hose

 

Persetrian Warning noise maker

 

Must be UK thing tuv or something .

 

Yeah looks like pedestrian alert. Pics aren't that good:


HORN. 2014 Toyota Prius Hatchback | Toyota Parts Center Online

 

But what's it doing way over by the starboard horn, instead of behind the fender on the port side?

 

The image is for the wrong model, actually, which highlights another regrettable deficiency of SimplePart’s catalog presentation on parts.toyota.com.

If you select 2014, Prius, and then set the Trim Level to Hatchback, you get catalog code 224420, for Prius Plug-in cars, which aren’t listed separately by model name. Notice how Figure 82-01, Battery & Battery Cable, in that catalog shows the larger HV battery.

If you set the Trim Level instead to BASE (VIN Starts With JTDKN3U), you get catalog code 222430, for liftback Prius cars. In this catalog, Figure 86-11, Horn, shows the vehicle approaching speaker assembly, part name code 86570, in the central location where it was photographed by @Ford Galaxie, with the note “※3 (1310–____),” indicating that this applies to vehicles made in October 2013 and later, that is, for U.S. model year 2014.

 

What trim is yours prius