For the past several years, I have often been called with only the caller's call sign. Only once. No my call nor DE before his call sign etc. Very simple. I noticed those callers always make the simplest style of QSO. No ragchewing at all. When they finish the message least necessary, they always finish their sending with BK withoug IDs. As soon as the message exchanges of least necessity are over, they quit leaving least greetings to me.
I noticed they might use a memory keyer. The keyer might send his call once. All the operator is doing is just pushing a button of the memory keyer. The other messages could be from the memories as well. He might be watching the display decoding my message. It is the same as RTTY or other digital modes.
I hope I won't guess right. But observing carefully of those hit and run operators, I must say there are pretty numerous guys operating CW in this way. I won't mercilessly deny and accuse them for their irregular CW operation. And there could be some hams who would become proficient in CW after operating in this way. However, watching conversations on CW may promise them much faster learning of CW and much more fruitful enjoyment of CW in the future.