Assume you are searching an area for a missing item. The item is static and does not move.
The probability of the item being in the area is 0.6; the expected search effectiveness is 0.4. Thus, the probability of detecting the item in a given search is 0.6 * 0.4 = 0.24.
Assuming each search is independent of the one before, how do you calculate the combined probability of detection for two searches? Intuitively, the more times you search, the better your chance of finding the item, but in some cases the searches might closely overlap, and in others there may be little overlap.