We have this on our 2015 car and its a sign of the boot switch failing in the handle on ours, at least. I have power washed it and soaked it in WD40 and its returned to more normal. Although it still plays up. I do anticipate having to at least remove the handle and get it cleaned and replace the micro switch. It seems to be a combination of age, fine road dust and water. There is a great post on here showing how to replace the microswitch. Its a fiddle but only requires a couple of wires soldering and some time and mastic. The alternative is a new rear trim with the handle and they are over £140 new.