The car rental .GPI file I had already removed from the Garmin\Poi folder, which removed the rental agency related POI's. It also returned the "Home" option in "Where to?" which had been absent prior.

The only *.img files in the Garmin folder of the device are gmapbmap.img (WW Autoroute Basemap NR v2.01), gmapprom.img (CN Southern Africa NT 2017.30) and gmaptz.img (Time Zone Map, v19). No SD card inserted in the slot at the side.

Continuing on the same thought as Cnfhbr and Butters, I replaced the Southern Africa map by an OEM map. Guess what... the rental agency reference is gone! Apparently the reference was embedded in the Southern Africa map.

Thanks guys for your suggestions.