I can't see any reason one of those screws would fall out, unless it got a bang or something.
There are three screws. Take a high power magnifying glass and look at the other two to see if there is any sign that they were removed, don't see how they could have been unscrewed with out leaving a trace.
The only reason for removing them would be to get at the front element. If it was professionally done, there may have been a reason. If amateurishly done, maybe to clean out internal dust.
Whatever, if the lens works perfectly, symmetrically sharp and you got it at a good price, you can just replace the screw and forget it.
