I am not sure either, but can you sing or whistle? Maybe it would give him more advanced warning.
How about going to the dumpster at the same time every day (as needed, of course)? If he expects you to show up regularly, it might not throw off his "autistic" routine, and he might be less startled at your arrival.
In any case, if you feel threatened, then maybe a word or two to the people responsible for the dumpster would be in order.
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The mere fact that science may not yet adequately explain an object, event, or experience does not mean the immediate explanation should automatically default to a conspiratorial, extraterrestrial, paranormal, or supernatural cause.