In those sentences, "there is" can have two distinct meanings. Which meaning is intended can only be determined by context. #1: It can be the equivalent of pointing your finger: "Look there! That's Sarah..." #2: (This meaning is valid but less common): "Sarah IS at the front desk...". Saying "There is Sarah" puts more emphasis on the existence of Sarah. The more normal way to say this is "Sarah is at the front desk..."

There is Sarah at the front desk if you need help. = Sarah is at the front desk. THERE IS basically means that something exists. . . . There is a blanket if you get cold. There is beer in the refrigerator if you get thirsty. It doesn't mean the thing is literally 'there' in sight at the moment you're speaking. There is Michael waiting outside for you. THERE IS in this case means that something is visible. It is often preceded by LOOK, as in, 'Look! There is a tiger.' Did you see if there is Alex in the library. This is incorrect, or at the very least very unnatural. You would say just something like, "Did you see if Alex is/was in the library?"