Santa Rosa is very sweet and juicy when ripe but at 1-1/2" they would have been green right? Any blush to them at all? I only have European plum trees which don't get all that sweet or soft, but squirrels will start taking bites and dropping them as soon as they turn the least bit color, and chipmunks will join in.
A dozen isn't all that many for the greedy thieves. The tree will also typically lose/abort some in early green stage -- called June or green drop. If this is a young tree or it was stressed in some way, it may have given up on maturing them. Is the area under the tree mulched? Mulched ground is easier to see what the tree (or animal) have dropped,
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