I'm pretty sure this was from our inventory at work. I say this, because I have the missing key caps in a bag over at the other office. I held out hopes the director would come through and get them to me (he said he would).
Maybe I'm missing something myself (my wife would agree), but I don't see any missing caps on this board. There are several
blank caps, where functions weren't used on this particular terminal—could that be what you're noticing?
Keys with missing caps are noticeably shorter than others, and there's usually a visible gap around them. Blank caps are the same height as keys with printed caps, with no extra gap. Example: