The first time it had happened, Beth didn't think much of it. It was after Merle came back to the prison announcing that the Governor has been taken care of.
Michonne was carrying the proof of His Highness's demise in a bag.
" What are you going to do with... it?" Glenn asked.
" I always wanted a fish tank" Michonne seemed to be genuinelly considering her answer.
Beth didn't want to look in the bag. Or at the bag, for that matter.
She crept over to where Merle stood, grinning at both Spartacus's remark and Shortround's reaction to it. That's why he didn't notice what Bambi was up to before her lips touched his cheek. It was just a second, but Merle noticed how good she smelled and how soft her lips felt against his scruffy face. He was observant like that. Must've been a Dixon thing.
"It was just a thank you kiss" Beth shrugged later, when Maggie questioned her sanity.
For his part, the prison's resident ex-Devil was too damn stunned to think anything at all.