It really depends on the context and how their relationship really is. If someone uses it once in a while sarcastically or jokingly its fine. However if the person doesn't use such intimate names at all for anyone else but uses it on someone who isn't his partner, its a little fishy.

That's why you also have to see how their relationship really is. Are they perhaps actually attracted to each other, but denying or not pursuing it for some reason? If that's the case, its just a timebomb waiting to explode. Otherwise, its probably fine.