I would be more concerned about the rust marks left by the seat frame ..you have a leak probably the lower rear corners of the rear window.
these trackers are for theft recovery and usually require an annual subscription in the UK
See Section 2 on the front page of your V5C Registration Document - the previous Registered Keeper. If they don't know, then it's just fit for the bin.
I dont think thats a standard product. Most likely it was installed by a previous owner or potentially the place that sold the car. A lot of used car places will install trackers so in case they need to repossess the car they can find out where it is. Strip it out and remove it because I don’t think you need it