While recording, the recorder doesn’t read the tape. There isn’t any sort of “magical marker”. The recorder simply tries to guess how much tape is left, probably judging by the rotation speed of the supply reel.
When you start recording, the chipset reads the tape for a short time so it can switch to recording mode at the beginning of a tape frame. Possibly it also synchronizes with the PASC frames but maybe not – it’s not really necessary because finding the start of a PASC frame is a matter of finding two bytes that are set to FF.
By doing this, the recorder guarantees that the time codes on the tape are continuous, so that a super user tape doesn’t change into a user tape. Once the recorder has switched to record mode, it’s impossible to read the tape. The playback heads are behind the recording heads so if it would be possible to monitor the playback heads during recording (which if I understand correctly, it isn’t) because the playback heads would read the bits that the recording heads just recorded to the tape.
When the recorder stops recording, it doesn’t stop the tape right away. It records a short marker that says what it did at recording time: change sides or end the recording. The “end of recording” marker (that’s not the official name but I can’t think of the real name right now and I don’t feel like digging up the DCC system description) is what the recorder looks for when you use the Append feature.
So there’s no way for the recorder to know whether the tape has reached the end, unless it has an optical sensor (which I think only 2nd gen recorders have!). Even if a recorder has switches to detect how long the tape is supposed to be, there’s still some inaccuracy about the actual capacity. The engineers wouldn’t rely on those switches anyway because that would mean that if any manufacturer would make any kind of special tape with some extra capacity would make it look as if the recorder is broken. That would be bad. So they decided to do away with the tape length switches altogether (saving money) and just let the recorders estimate how much tape is left. And in order to make sure that there is time to record a market at the end of the recording, there must be a bit of a safety margin.
In a controlled environment (like making a recording of a known length on a tape that has a known length) it would be nice to have a workaround and force the recorder to record the whole thing. Maybe it’s possible to make a hack. I wish I had more time.