In literary/cinema mediums, when a character extends their hand in friendship while hiding their other hand behind their back, that can actually symbolize tension between trust and betrayal
The hidden hand means the character is hiding something, literally or metaphorically. A weapon, a scheme, an ulterior motive. It signifies that their friendship is likely a trap
TLDR Maddie was always hiding something and the writers hinted at it the whole time
Fuck that bitch leprachaun