Minicanon!
During the Family Diner era, with Fredbear, Spring Bonnie and Spring Shine, "My Girl" by The Temptations was a song that would occasionally be performed by the trio.
Spring Bonnie wasn't always one to sing along with the other two, more focused on providing the musical accompanyment. Doesn't mean he never sang, though. And when it came to "My Girl", he very much did join in on the singing.
Though, for the longest time, Fredbear didn't realize he was singing as third wheel when it came to performing this song. He would just be happily singing and performing it like normal for the Diner's guests, meanwhile, Spring Bonnie and Spring Shine would be intentionally harmonizing together with clear air of wholesome chemistry in their performance.
Once Fredbear clued in, their performance for that song was changed around a bit to be one done more by Spring Bonnie and Spring Shine.
And yes, they'd get a healthy amount of teasing comments from Fredbear after store hours, but those never bothered them since their relationship was never a secret in the first place.