Menachem linked to the Tzitz Eliezer, volume 5, responsum 1. Therein, chapter 5 discusses, among other things, our question. He writes (in my own very loose translation):
As was explained in the previous chapter, there's seemingly no basis for a requirement to fast imposed on those who saw a sefer Tora fall.… Nonetheless, many great pos'kim have required that not only those who saw the sefer fast but also others who pray in that synagogue and live there who were not even present. Let us examine the basis for this and what other pos'kim hold.
Ikare Dinim (OC 2:15, citing S'de Haaretz 3:31) has:
If he didn't drop it and didn't see it fall, but saw it on the ground, there's not even a custom to fast; one who does so should be blessed.
… Olas Yitzchak 67, after explaining the lack of basis for fasting altogether… adds at the end that
Jews are holy and fast even if they saw the sefer fall from another. I'll even add a further stringency: in our case, where they didn't even see it fall but saw it lying on the ground, they should fast one day of B'hab…
and the like is in Chazon Nachum 86 who requires a fast of someone who came across a sefer Tora lying in disgrace on the floor, as well as the sexton and gabaim.
However, Yad El'azar 126 took for granted that those who come across a sefer Tora lying on the floor need not fast, since they did not see the fall. He did require that they 'redeem' their fast with some nominal charity for the poor; and he did require a fast of the sexton or officer who could have set the sefer in place and closed the cabinet properly.
What we get out of all this is that there's a great basis for not requiring any fast at all, not even as a mere stringency, of those who did not see the fall, even if they saw the sefer Tora on the ground. Also, even those who wrote that those stringent on themselves should be blessed wrote that only of those who saw the sefer on the ground in disgrace.