So, it’s early February and Len is thinking about what would be a good gift for Kaito’s birthday. He probably ends up acting unintentionally suspicious so one thing leads to another and the girls (most likely Miku and Rin) are pretty convinced Len is actually sneaking off to buy a Valentines gift for somebody and are *determined* to find out who he likes. They get so invested into finding out that they pretty much forgot that Kaito’s birthday is coming up and don’t even consider an alternate explanation. After not seeing Len buy anything that would be Valentine’s related, they give up, although slightly frustrated, probably thinking Len chickened out. Then comes February 17th, Len is carrying a birthday present for Kaito and as soon as the girls see it, they freak out, maybe even tease him for being late to have enough courage to give a ‘Valentine’s Gift’. Len just ignores them and walks over to Kaito, which causes further freaking out from the girls, until he hands him the gift and wishes him a happy birthday, Everything clicks into place, they realise what he’d actually been shopping for and can’t tell if they’re relieved or disappointed (and that they’d totally forgotten it was his birthday).
And as with a lot of people who have birthdays near other holidays, especially gift giving ones, I can imagine Kaito being so thrilled that someone actually took the time to remember his birthday and get him a gift.