Never by a total lack of evidence. Sometimes the evidence leads to a few possible conclusions, and on those occasions its necessary to try and get a 'gut feel' for a situation.
As an example, on this trip I so boringly wrote about in the OP I was investigating an allegation that security officers had used undue force to restrain a patron and in doing so badly fractured his arm. This was after he had allegedly been head-butted by an off-duty security guard. I interviewed the witness nominated by the claimant, who admitted he is a close friend, and he gave me a version of events pretty much identical to what was alleged in the claim. I also interviewed one of the security guards involved, (and the son of the licensee), as well as a bartender who had been working on the night in question. They told a very different story, in which the claiamnt was the aggressor, throwing punches and hurting his wrist whilst flinging himself back onto the bar whilst held in a wrist-lock.
No CCTV footage because it was an adult entertainment area. Interesting. What actually happened?
The police were pretty willing to chat with me on this one, and confirmed that they investigated the matter from a criminal perspective and concluded insufficient evidence to proceed with charges. One of the things that threw doubt in their mind was lack of physical evidence that the claiamnt had been head-butted. However, the detective also told me off-the-record (on the basis that I only report the information verbally to my client, and in generic fashion) that this particular security guard had been in trouble for years because of similar incidents, and although had never been convicted, general feeling was that he was hot-tempered and skilled at covering his tracks.
My factual report to my client? (my client is the liability insurer for the hotel). That's easy. I present the various different accounts, include the official police report, provide photos of where the incident occurred etc etc.
My opinion on how the insurer might best proceed? Tough to know. I'm still weighing up my personal impressions from having met and interviewed the security guard. I need to investigate any past licencing breaches (one was admitted, the police hint there may have been several). I would say, probably, that I'll recommend the claim be 'setttled on best terms'. It's one that would be risky to take to trial, bearing in mind that a judge would not have to be factually convinced to make a decision that the hotel had been negligent.