Is a conversation between two colleagues or a group in a lab meeting considered prior art? Neither are in a public forum. Is there an assumption of confidentiality?
I'm not a lawyer, but my understanding is that something has to be publicly disclosed to be considered prior art. Conversations between colleagues within the same business is clearly not prior art. What conversations with other employees does impact is inventorship. If someone contributes an idea that gets reflected in a patent's claims, then they probably should be considered a co-inventor.
This is a very interesting topic and a short answer is rather impossible. It's all about how you define the public and its access to prior art.
The EPO (Europe, for the most part of it) considers that "information is generally to be regarded as having been made public if even just one single member of the public is in a position to gain access to it and understand it, and if there is no obligation to maintain secrecy". What happens, though, in practice is that such guidelines driven by the case law is a dynamic set of rules which changes with time. Some examples are clear-cut cases and some are subject to new-created case law. For practice as per EPO you can read more here: http://www.epo.org/law-practice/legal-texts/html/caselaw/2016/e/clr_i_c_3_3.htm
For the US I am not fully aware of the rules that regulate such issues, but there should be relevant case law there as well (who is "public", the concept of "secrecy"). Mind that in the US there is an exception, a one-year grace period, within which a disclosure made by the inventor or someone who got access to the information by the inventor, is not prejudicial in a patent application's novelty (US experts may correct me here, if I am wrong). This exception does not apply, though, in other jurisdictions, particularly the EPO, so "handle with care".