In general, you cannot search for arbitrary temporary positions in a video file format such as MP4. This is because video players rely on the container to provide file offsets for video frames that can be arbitrarily accessed, and only certain frames are qualified. (Most video frames cannot be decoded on their own, but rely on information from several previous frames.)
However, many videos provide the illusion of finding arbitrary time positions, providing many points of random access. In other words, the GOP size remains small (key frames every 1 second are typical), and these key frames are mentioned in the MP4 structure. Your videos are probably encoded with large GOP sizes, so they have very few search points.
I suspect that if you transcode your videos using a tool such as FFmpeg with the default settings, you will find that they are much more useful when searching.