Inspired by Dave L Renfro's answer here and Interpreting the statement "We received an extremely strong pool of applications this year." in US grad school decision letters
If I apply for a postdoc (or tenure-track, etc.) position, fail to get the position, and am told I was the 2nd-choice candidate, can I reasonably assume that I am indeed the second choice, or is that something departments do to soften the blow of getting declined?
Competent people involved in faculty hiring in some official capacity (say as department chair or a member of the hiring committee) will never outright lie to a job candidate, even for well-intentioned reasons. If they want to hide certain awkward truths they will instead use vague or diplomatic language. It is then up to job candidates to interpret what the vague language means (there are many questions on this site giving examples of this). But the general rule is that such vague statements usually do not contain more information than their literal meaning - they are crafted precisely to have that quality. So yes, if you are told that you were the second-ranked candidate, I’d say there is a high chance that that factual statement is in fact true.
Two caveats to keep in mind are:
It’s still not a guarantee that the statement is correct. There are incompetent people who deviate from normally advisable ways of doing things, so it’s certainly possible that a chair will lie to a candidate, and probably happens every once in a while. (This would probably be correlated with other weird or alarming things the candidate might have noticed about the chair’s behavior.)
If the candidate heard that they were the second-ranked candidate not through a formal channel but through some informal gossip, say through a friend on the faculty who isn’t an administrator or on tbe hiring committee, the possibility for lying (especially a “white lie” told out of a misguided desire to protect someone’s feelings) seems more likely. Telling such a lie is still a stupid thing to do in my opinion, and still creates many possibilities for real harm as well as legal liability for the university, but professors do not always have common sense about such things, especially when they are not themselves administrators and are not sensitive to some of the legal issues related to hiring.
For a tenure track candidate for a position at a large university that uses typical recruiting practices, if you haven't been invited for an interview, you're probably not the second choice, even if you were told you were. This probably includes distance interviews these days.
Beyond that, it can be really tough to say.
As other people have said, they should definitely not lie about something like this.
However, as far as I know it is not normal to tell someone they were second choice in the first place. So this is an unusual situation and I wouldn't be too confident that the usual norms apply.
The exception would be if there is a good reason for giving out this information. For example, I have heard of people being told "you are the second choice, and the first choice has not yet decided whether to accept". This seems like helpful information to give (if true), and there is no good reason to lie about it, so I would be inclined to take it at face value. The probability of subsequently being offered the position is perhaps not high, but I have heard of it happening.
Lying to a candidate is in no way helpful to them, so telling them "to soften the blow" is totally misguided.
You tell them they came second if they actually did, and if you had been willing to hire them if the first candidate hadn't been there, to keep a possibility open to hire them if (a) the first candidate rejects the offer, or (b) you suddenly get more budget and can hire two people, or (c) you want to pass their name on to someone else who is hiring.
And you tell them they were good enough to get the job, but one (or two) people were better, if that is actually the case.
