No signal. Safe is reserved for the end of a play at a base. Except dropped fly balls or line drives that is..
There is no signal made. Not to say the over zealous might want to communicate to all that a tag "say up the line a bit" and the runner having not scored yet was missed, it isn't necessary as the runner isn't safe yet. Perhaps a verbal, "nope" or "missed him" to inform the players directly involved but no safe signal.
1. when the play isn't done yet, ex., missed tag and missed plate, F2 trying to tag the scrambling back runner, we ain't done yet, you don't get a signal til it's over.
2. Missed tag and runner continues on to the dugout, there's nothing to signal he scored (he wasn't tagged and he passed the plate), there's no signal given for a runner scoring unless it's a time play.
A good example, is R1 and GB to F4 who scoops it and attempts to tag the passing R2. He tags R2 you get an out signal, F4 misses the tag you get nothing cause the play isn't over.