If you consider Tobi's explanation to why Minato incorporated the mark and the word ALERT, it will connect to him being able to sense danger. He incorporated the mark so he will always be there to protect kushina. Take note of the word protect.
QUESTIONS:
1. Why would Minato incorporate the mark if he's always on her side?
Since he can't be always at her side, he incorporated the mark so he can help her in times of trouble. It make sense right?
2. How will he know that Kushina is in trouble assuming that they are distances apart?
The mark alerts him. This was being mentioned by Minato himself.
It's very clear that the term alertconnects to "sense danger" if we base on how Tobi explained it and how Minato described it.
if kakashi picks up the kunai, he's definitely alerted, because kakashi is fighting and, unusually enough, using the kunai.
how about he incorporated it so that he can check easily? maybe kushina even has a way to tell him, like what kakashi did when he picked up the kunai?
he said that the mark on "THIS" kunai alerts him if it "FLIES". He did
NOT feel tobi at any point in time, and i'm sure that the kunais minato carries have ftg marks. He did
NOT save kakashi when he lost his eye. He
CANT feel danger, especially not via his mark.
dont ask others to read the text freely and afterwards, when your theory is proven wrong (the kakashi-example is a clear counter-example) demand the one word that might back up your theory is interpreted in the only way that would be useful. every interpretation like "he has the sign in her seal to be able to get fast to her, so that he can protect her" works at least as fine for that translation as "he has the sign that can sense danger so that he can protect her when the sign tells him".
dont just go and say: that can be interpreted in both ways, so he has to have an ability that he never clearly showed and sometimes didnt use for reasons that nobody would ever understand. that doesnt make sense. stick with what the characters show.
i dont go in a sasuke vs naruto thread and post: "sasuke can evade all of naruto's attacks because he has the ems and that enables him to see all of naruto's attacks. apart from that, since itachi killed some clones behind him and therefore can sense danger, sasuke probably has that ability too, so sasuke cant be surprised" just because i like sasuke more than naruto (which is not the case in minato vs itachi, where i like minato more than itachi, because itachi is a douchebag). it would be just something that could work out in my head, but there is no evidence that that really works, and it isnt probable.
i'm off to sleep, good night.