Whenever someone talks about senior engineers, two close friends spring to mind immediately, one of which is my brother (software development).
Even Though I am convinced that experience (and thus age) contributes to the segregation between engineers and senior engineers, I also think that it is a special frame of mind which enables some people to truly become the top of the crop. Despite my inflated self image and overly optimistic assessment of my own intelligence, and despite the fact that I am convinced I would have little trouble convincing an f500 company to give me the title of senior engineer, I know that I can never attain the 'seniority' my brother and friend already possess.
Having scanned through the responses, I didn't immediately see the specific behaviors I have found in top engineers. First of all, I disagree with the humility trait. I would not call top engineers humble. I would not call them arrogant either. Top engineers have strong opinions, yet are flexible. They have their ideas about best practices, yet are 100% comfortable adopting something else.
Using a stupid example in software development, tabs or spaces. Number of spaces. Top engineers will have their preference, whichever it is. And they have thought about it, deeply. Not just from their perspective, but from the perspective of all engineers, future, present, and past. You may probe as deeply as you want, they will have looked at it from every possible angle and will be able to explain to you in every detail why they prefer the one over the other. And! This is so, so important... And! They will put their preferences immediately to the side if that's just not how things are done on this particular project, in this particular team. I wish I had better words to convey this thought...
Top engineers will work with what they have to get to where they want to go. And, that's not just the technology, it's also the team. Senior engineers in your team will automatically make everything better. Sometimes their value is educating the entire team on best practices, sometimes it is just driving towards success despite the all of the feces.
Sometimes they might come over as arrogant, because they can speak with confidence on certain topics. That's another thing that sets top engineers apart from junior and regular engineers, whenever they speak with confidence, it is because they are in fact confident. And they are only confident when they have analyzed an issue sufficiently deeply and from everyone's perspective that they are comfortable being probed deeply.
When they do not speak with overzealous confidence, it is because they are still learning about the subject. And they will learn and analyze it deeply if that is necessary.
A single inconsistency, or a single counter example, or even a single ambiguity is sufficient for them to reconsider their position completely.
Also, senior engineers will never assume they know anything. They tend to listen attentively whenever anyone is talking. When someone says something 'stupid' they won't assume that the person is stupid, or said something stupid, they will assume they did not understand something and ask for clarification. They tend to not jump to conclusions... If something is ambiguous, they will identify it and ask for clarification. As a result, they will often ask for follow-ups in the future... such as: "Thanks for the information, can I get back to you if I have more questions?" Now that I think of it, I don't think I have ever seen someone whom I consider to be a top engineer not keep that door open....
I hope this helps.