If your boob-filling leaked, what can happen?

Depends... ...On the type of implant. If it's a saline-filled implant, the body absorbs the salt-water without any problem, and you just notice a "deflation" of the breast. If it's a silicone filled implant, it's harder to tell by looking. You might need an ultrasound or mri. But still, not a lot happens - if you leave it for a long time you would probably make a lump called a siliconoma. See your md.
Deflate if saline. Saline implants will deflate if they are ruptured. That is one of the benefits of saline implants. If you had silicone and suspected a rupture, you would have to go for imaging such as an MRI or ultrasound becuase most ruptured silicone implants will maintaine their shape which is known as a silent rupture. Failure to treat a ruptured silicone implant may lead to foreign body granuloma formation.