Before reading the text sorry for adult content. I have my doubts if the word " plant" has to do just with putting a kiss or putting a hide illegal kiss. Both definitions are from Longman.
After a lot more sniffing, I decided to plant a kiss on her boots. I kissed the inside, then the heel, the sides and finally the part for the toes.
To plant something is to deliberately put it in a specific place.
Therefore, to kiss her boots is a more casual act and to plant a kiss on her boots is to place the kiss deliberately and precisely.
"plant a kiss on something" is a common expression for "kiss something", with a somewhat more gentle, more romantic connotations. Nothing about hiding or illegality about this one.
("planting evidence" is different, meaning placing incriminating items in possession of someone, e.g. putting drugs in their car in order to "discover" it later during a search.)
