It is actually a complex word. "Être témoin (de quelque chose)", without "un" actually meant that you witnessed something. "Il a dit ça, je suis témoin" = "He said that, I witnessed it". We don't have a verb for "to witness". Otherwise, it also works like a profession, if you say: "Il est témoin dans une affaire de meurtre" = "He's a witness in a murder case".
No, it does not mean "I second that". It is not a verb; it is a noun. The reference to "second" is in the sense of a witness (at a duel)! http://www.larousse.com/en/dictionaries/french-english/t%C3%A9moin/76291 Duo also includes "warning light" as a hint for témoin. As you have heard before, it is unwise to take Duo's hints at face value. Check first and verify that it really means that and that it fits in the given sentence, i.e., it carries the meaning of the original and presents that meaning in English in an articulate and correct way.
Except that in English, without any other context and without any further words of explanation and without revealing last minute backstory like Perry Mason, you need to include the indefinite article "a". I am a witness. You will recognize the "Je suis (noun)" structure as meaning "I am a (noun)". [Edit: See comment by super_moi, above, "It is actually a complex word."]
I heard "téman/témen" which doesn't seem correct. It's possible that the computer's accent is the problem here, since nasal sounds do vary from the more "correct" pronunciation. For instance, here in Québec the "on" sound often sounds like "an" (French pronunciations, in all the above).
Actually, there are all sorts of professionals who are exactly that. They are called "expert witnesses" (l'expert). They may be scientists or forensic experts who examine evidence and then present their findings to the court. E.g., ballistics experts, blood experts, handwriting experts, etc., etc. http://www.larousse.com/en/dictionaries/english-french/expert_witness/579500
These are categories of experts not categories of professional witnesses. To include these under the banner of "witness" is incorrect in my view because each of those individuals are experts in their own field. None that I know of started or exist totally as an expert witness, because each needed to gain significant experience in their particular field to be regarded as an expert. So I would describe them as "Expert Ballistics witness" or "Expert Handwriting Analyst witness" etc. But perhaps this is semantic.
An "Expert Witness" is better but still ambiguous. But I don't believe there is a such a thing as a "professional witness" for the reasons I've given. If we regard professions as categories - such as medical doctor, or builder, or mechanic - possessing a distinct function and purpose, then there's no such thing as a generic professional witness. I know of no-one who introduces their profession as "Witness". However, each of the above would differentiate their field in the title. "Witness" as a class of profession is simply too ambiguous in my view. So, no. I would not agree that there is such a profession as a "Witness".
Re: "An Expert Witness is better...." This is a straw argument by putting forth the term "professional witness" and then proceeding to shoot it down. The reality is that the sentence being discussed, "Je suis temoin", is translated as "I am a witness". There is no point arguing about whether someone's profession can be "a witness" but look at the French and understand why the structure "Je suis temoin" works here.
Of course not! My view was never a critique of the sentence structure or grammatical lesson. If one understands that the intention is to classify a witness as a profession then they will get that the structure is synonymous with expressing professions. There is a point in arguing whether such a profession exist though, as it will be relevant if this is actually regarded and used by French speakers (although I'm unlikely to use it proactively).
As for making a straw argument.... :-D