Flash sensor

Every once in a great while the red sensor on my flash shows up in the photo. For an example. I took a picture of my husband and when the flash went off the red light covered his face. why? It will be in a low light situation.

Thats not the sensor. The sensor is inside the camera. It would be helpful to mention what camera and flash you are using.

Could be the focus assist beam, but I'd expect that to have died off before the shutter actually trips. Alternatively, if you're using E-TTL IR to control off-camera flash units it could be the master communicating with the slave units. Again, I wouldn't expect that to affect the picture, unless you're using second-curtain sync. You'd probably also be using P or Av (with the default, shutter speed auto, setting) mode In either (or any other) case, the low light levels would be the key factor; you may be getting the same event happening in brighter lighting conditions but the ambient light is masking it.

Show us an example or two, with the shooting conditions and camera settings.