I am not sure.
if you do not feel unsafe, does that automatically make you feel safe?
No. ^^ More like 'neutral'.
.. is there something additional that makes for safety - something definable in its own right beyond not feeling unsafe.
I think so. ^^
As others have alluded to, safety/ unsafety may be as regards physical harm, but I think also emotional, psychological, financial, and in other ways. I am not good at words, but I would say 'unsafety' (for me) involves a lived awareness of the possible consequences or fall out, knowledge of the potential long and far reaching effects or debilitization (and not feeling able to go through it again); the loss of more of what makes one's self 'themself'. It is far beyond relief, and greater than anxiety in the way of feeling or seeming to be on the cusp of occurring, and can't be prevented or avoided. Also perhaps an element of not knowing how bad it's going to be, or expecting it (should it occur) to be devastating. Or the consequences to be devastating, including decisions made or trust given before realizing the danger was there.
(I do personally feel since nothing can be predicted, or anything can happen at any time, there are no guarantees of safety in any given moment, but that is life.)
I can't really describe safety, except to say maybe the opposite of the above: having a sense of resources (and practical ones, +/or deterrents or stop-gaps ) when in the path of physical dangers; financial resources or physical resources (people), to help when one cannot help themself, or to turn to or provide safety; predictability, with earned trust, for safety from emotional harm (integrity of the people around, and care vs harm).
I think relief to me is more like getting home, when it seems it's impossible and the day will never end, and the gratitude that comes with that. Whereas safety is more like the feeling that comes when you don't believe it;s possible to get home, but you realize you are home, and that home (and you in it) are free from all concerns of harm. So I suppose it means feeling protected, secure, and free from doubt and debilitating or horrible worries of the present or future, based on what's been learned from the past. Like being in a rowboat in a storm is a very different feeling (and reality) than getting to a dock (if it's not submerged), and different again to getting in to a secure shelter. Or versus there being no storm at all. Whether that be physically, psychologically, +/ or emotionally.
I'm sure this could be said in a shorter or simpler way. :( Knowing in your heart it's ok, or because of who, what, where you are or what you have it will be ok, and truly trusting in that. That to me would probably be safety. A feeling of relief in, or confidence in protection from harm, in all it's forms.
Which written like that, sounds pretty unrealistic. But some things are impossible to control whatsoever; some things are bad luck; some things are not within the realm of immediate change (financial resources, or family). Other dangers are interpersonal, and some are because of lies or betrayal. Safety to me is also when those are not present, so it may be fleeting but is still real, at least for a (those) moment(s).