Most of the search and rescue missions I participated in began after dark because that is usually when the call for help came in about an overdue party. Having spent a great deal of time looking from both the air and the ground for people in trouble, I can tell you night time searches are much easier and the chances of finding someone much greater. Fires, flashlights, and emergency strobe lights all show up fairly well at night. If we are using night vision goggles then they stick out like a sore thumb and are hard to miss. In fact, I have found people by the flash from our anti collision strobe lights bouncing back off the reflective trim on their shoes or packs using night vision goggles. Gen 3 NVGs are that good. However, at some point, the sun always comes up and that can be a problem if we have not yet found you.
Daytime signaling is an entirely different story. It is a big problem because there is so much out in front of or under me that it is very difficult to create enough contrast between the target and background so that I can see it unless I am looking directly at it. Even then I could miss it if a tree, rock, or wave gets between us at just the wrong time. Then there is the human factor that a lot of SAR guys don